Yoga Sutras of Patanjali by no means comes as the most famous and the most quoted primary source on Yoga. I know about 20 translations of Yoga Sutra into Russian and ca. 50 into English (those who want to see the list may refer to Schedules 1 and 2). The exact dating of YS is still the matter of dispute between the scientists that I would not quote here. Those interested can find the points of view and arguments drawn on this topic in the works of Nikolaeva, Verstein and others. There are also a zillion of myths about Yoga Sutra that are obviously shared by incompetent people but that are nevertheless rather adverse. So I will start with debunking these myths.
1. Yoga Sutras is not the only primary source on Yoga. There are at least 150 classic yoga treatises known to me. And probably no one knows how many of them there actually exist. It is rather that Yoga Sutras might be the most advertised source in scope of the European culture. Maybe because it was among those first translated into European languages. Maybe because it was admired by the luminaries of modern Yoga, such as Krishnamacharya and Sivananda.
2. Yoga Sutras is not the most ancient work on yoga. The practices of yoga were described, for instance, yet in Mahabharata, which is at least several hundred years older than the Yoga Sutras, as well as in some early Upanishads.
3. Yoga Sutras is not an exhaustive description of yoga. For instance, here are the notions already existing at the time when the treatise was written that are yet not mentioned there: strengthening asanas, dynamic practices, mudras, kriya and organism cleaning procedures, chakras, the majority of meditation practices, tantra and sexual practices, bhakti- and karma-yoga, the techniques for raising the Kundalini, the practice of conscious awareness, etc. And if we analyze the modern stock of yoga that’s been significantly supplemented by archaic, Tibetan, Taoist and Sufi practices which are not mentioned in the Yoga Sutras this will make the list even longer.
4. Yoga Sutra is not an entirely original work. Some inclusions contained in chapter 4 can be obviously traced back to Buddhist concepts that incidentally do not fit with the main text.
5. In terms of logic Yoga Sutras does not come as a completely solid piece. When subject to thorough analysis, some concepts appear to come into collision with each other. Unlike the axiomatically set Buddhist texts YS rather resembles the recommendations of an experienced man of practice who understands that in terms of his text different followers will find different advice.
The above said may seem to come as criticism of Yoga Sutra but it is not. Yoga Sutra is a brilliant piece of work that is great both in terms of concise writing and in the number of ideas that were totally innovative in the history of esotericism. It was not by accident that I have put here "esotericism" and not "yoga" because I consider the global esoteric idea to be a unique universal phenomenon. Different systems were being completed and adjusted by each other’s experience and the ideas that were formulated in one tradition unfailingly occurred in those others. So here are the ideas that were originally formulated in the Yoga Sutra:
1. The spiritual advance of the person is of psychological and energetic nature.
For the man of the day who was originally brought up in scope of the post-psychoanalytical culture this idea initially seems apparent. But in fact it is incredibly advanced in its basic essence. If we take the descriptions of yogic practices and asceticism drawn before Patanjali we might see that their main accent was laid upon the façade of the event. The ascetic accumulated tapas but he still remained the same person with the same weaknesses (and there is a number of myths on this topic); the Brahmin studied the texts but the resulting knowledge was again performed for the sake of the external form – for fulfillment of requirements or proper performance of the ritual, the bhakti ... It was, in fact, the behavioral and not psychological approach that was determining all aspects of life prior to the Axial Age. It is not by coincidence that some researchers refer to the period from 2 thousand BC until the beginning of our era as the period of Law. It was then that all strong cultures laid down their rules and regulations concerning the social behavior of a man: there appeared the Code of Hammurabi, the Laws of Manu and Moses, etc. The religion was also guided by human behavior and not by feelings and concerns the person had inside. The advance was first and foremost understood as setting the person’s behavior. And so the crisis of such approach started to reveal in many cultures. It was expressed in works that became crucial for they were describing the inner concerns of the hero who was suffering due to his inability to go beyond the set behavioral limits. These works include the Bhagavadgita, the Greek tragedy, and in some degree - the Epic of Gilgamesh. But there is a long way between understanding the recessionary nature of behavioral approach and spiritual practice goals’ formulation in scope of psychological terminology. And though we cannot say that Patanjali worked this way himself, for there were the predecessors and we will speak about them in the next section, yet he was the first one to clearly formulate this idea and the hence resulting consequences.
2. Karma is of psychological nature.
Prior to Patanjali – and they still continue to think so in the profane layers of Hinduism and pseudo-esotericism – karma was believed to be the retribution for the performed deeds. Many religions, including Christianity, contained the concept about every act of a man as observed and evaluated by some specific creatures, and sometimes even recorded in a special book so that later these (or some other more specialized) creatures could give a person his due.
In this aspect the Vedic culture adhered to some similar concepts and in order to check it out one can merely read some chapters of Mahabharata or the later Garuda Purana. The naivety and philosophical unfoundedness of this view is obvious. However the existence of the cause-effect relationships between the human deeds and his lifestyle is also easy to observe. The refusal from consideration of karma as a factor that is external in relation to person and its correlation with human emotionalism, with man’s intra-psychic nature and his desires – both conscious and not – came as a drastic step forward.
3. The spiritual advance includes both emotional and cognitive aspects.
4. The concept of advance as purification of one’s nature and complementary concept of advance as complement of one’s nature.
If we analyze philosophical base of any esoteric system of psychological practices, or even a plain system of psychological trainings, we will see that each of them is based upon either of the two afore-mentioned models of human development concept. In scope of the first model the person develops by means of “purifying” himself, his mind, his unconscious, the nafs, karma, the subtle body, etc. from either klehsas or fits of passion, engrams, the chains of maya, psycho-traumas, atoms of suffering, corporal bonds etc. In the reduced variant the followers of the purification theory perceive one’s body from the same very position trying to purify it from toxins, clenches and so on.
There is another model of development that comes down to the concept that advance comes as a process of acquiring by a person of new experience, new skills and competence, mental states, merits, good karma, tapas, etc.
One may easily see that the two main schools of modern psychology that are related to psycho-trainings, the behaviourism and psychoanalysis, are separated in compliance with this very aspect. In terms of behaviourism human psyche comes as a tabula rasa where anything can be written so that, in fact, the person can be taught some new behavioral, emotional and cognitive skills. Psychoanalysis (as therapy) perceives the established psychological type as a result of already received experience (sometimes disharmonious) that the person has to become aware of so that it could actually throw off its pressure . Of course in terms of reality the philosophy and reasoning of each of the schools is much more complicated, but in fact all modern psycho-trainings are in one or another way based upon one of these models. The person is either taught the new forms of response to problematic situations or helped to get rid of predominant psycho-traumas, habits, etc.
As a rule, every School tends to employ methods derived from one model; besides, very few practitioners think about philosophical background of their activity and its eventual alternatives. The greatness of Patanjali lies in the fact that he was the first to conceive and represent the two models: in an explicit form, concurrently and in scope of one and the same system. Moreover, he also determined practices deriving from each of the models.
5. The inner observer.
The concept of the inner observer, namely, of the innermost part of the psyche that one can be aware of, though it is a difficult thing to do, makes up the basis of any strong psycho-practice - from classical yoga to psychoanalysis of today. Indeed, one can work on change of one’s scenario and behavioral forms, emotional responses, even world-view and mindset. But there must be something we could be based upon. Some part of our mind that is not identical to all the afore-listed. And this is the function performed by the inner observer.
In other terms the inner observer comes as a prerequisite for the state of awareness. It is not that everybody has the feeling of the inner observer. The majority of people are fully involved into their scenarios and concerns. Nevertheless, the spontaneous awakening  (the term is not random ...) of the inner observer is possible in stressful situations related to existential crisis, namely in the case when the habitual role sets no longer work or come to collide with each other. In such situation the majority of people proceed with developed neurosis, yet some may experience the specific existential feeling of relaxed observation of the situation that was previously perceived as a real drama and realization of different ways one may act in its scope. The diversity that goes far beyond the usual forms. This is the state that brings new solutions and – as sad as it sounds - new scenarios. If they are successful, one’s life will get better and the experience of the inner observer shall gradually subside. But even such brief experience is positive because it has enabled the person to feel its real self at least for a short period of time.
Sergey Lukyanenko has a brilliant fantastic piece named "Spectrum". It describes the journey to different extraterrestrial civilizations; the specific feature of one of them was that despite the apparent high level of culture the adult members of the society were not able to develop. They were performing the social functions they had learned as children. In terms of non-standard situations they used to have the brain freeze being unable to take non-standard decisions. So how far does the situation go from the one we have? Are there many non-standard, truly bold decisions taken by an adult that were not programmed in his childhood? The decisions that cause a drastic change in his life. How often does it happen with the man of today that he seeks to develop fundamentally new skills when an adult?
As it has been already said, it is the presence of the inner observer that provides for fulfillment of the true spiritual practice, i.e. actual change of one’s behavioral forms and psychological responses. Patanjali refers to the inner observer by using the spacious word drashtar – verbatim, the judge, the one who makes the decisions. In this respect the concept of YS is much more searching than some of more recent approaches to the problem. It is even better that the term of the inner observer adopted in our Tradition since it initially highlights the active role of this part of one’s consciousness. Such understanding of the inner observer role brings it close to Heidegger's Dasein.
To be precise we should say that a similar idea had in symbolic form existed yet before Patanjali. It was already in the early Upanishads that they used the metaphor of man as a cart. His body is a cart, his feelings are horses, manas (intellect) is a coach. But what is the point in the cart if there is no owner? In fact such owner is this very inner observer  However, Patanjali introduced the concept of drashtar almost into the very definition of yoga thus denoting its significance.
6. The concept of rational planning of one’s spiritual advance.
7. The concept of harmonious SA and complementarity of methods.
8. The criteria of reaching Samadhi and its types.
9. The idea of the flow.
10. The role of altered states of consciousness in the process of transformation.
For those who have read my book about psychological practices - there is a separate section in the book dedicated to this subject.
Patanjali has at least two shlokas that specify the necessity of the ASC and mention the property of such experience to bear the sense of confidence in itself that was later represented by W. James.
11. Postulation of mystical experience as a valid means of cognition.
In some traditions mystical experience as a specific means of cognition is not allowed.
12. The feasibility of psycho-technical cleaning of karma.
People always had the idea of self-purification aimed at clearing off the consequences of the acts that are improper in terms of cultural behavior, as well as corresponding rituals. Yet it was for the first time that there was described the idea of changing one’s karma through some internal reorganization and not by means of different rituals such as clogging the oblational goat. Of course, being precise we should say that the idea of performing an "internal ritual" and "inner-asceticism" as opposed to the outer one had been proposed yet in the Upanishads. It was also Buddhism that referred to desires as the source of karma. But there is a whole gulf between the concept and its particular application in form of a technique.
13. The concept of subjective tinge in perception of the objectively existing environment.
“Objects are either known or not known according to the way in which the coloring of that object falls on the coloring of the mind observing it” - says Patanjali. This is a very advanced idea especially if we take into account that it was formulated two thousand years ago. Yet the same event looks different when observed from different points of view, and different people see one and the same event in a different way. What one sees as a reward the second may have for punishment and the third may think to be a problem. Today this idea seems common to us but it did not seem so obvious two thousand years ago.
14. It was Patanjali who introduced the basic concept of Jnana-yoga –the neti neti meditation.
Neither this nor this – the disidentification of self from all eventual identifications.
15. The basic idea of psychosomatics that the body is linked to the mind can also be traced back to the text of Yoga Sutra.
These ideas are considered to have been invented in the 19th century - but no, nothing of the kind, it happened two thousand years ago and almost point-blank.
16. The transition from working with air in terms of doing pranayama to working with energy.
This idea is also not that obvious.
17. Principal feasibility of disintegration with one’s worldview.
Go out and ask a person if he can disintegrate with his worldview, and 99% of those asked will not understand what it is all about because the person believes that what he thinks is how it actually is. Only those people who have been purposefully working with their mindset will be able to analyze that this is just a vision that can be viewed from a different angle, can be doubted or disintegrated with. In the work of Patanjali this concept is represented “point-blank”.
18. The concept of sub-personalities and the method of their synthesis.
It was invented in the 19th century in scope of European psychology. Yet we may find its prototype – the “auxiliary chittas” – in the work of Patanjali.
19. Comprehending the need of the soul’s incarnation in the matter for the purpose of its advance.
It comes to differ from already existing by that time pessimistic concept of Buddhism regarding the soul’s pointless staying in samsara inside the matter and endeavoring to get out of there.
Patanjali formulated a different concept that is much closer to the one proper of our school: the soul must exist within the matter because there is no other place for its advance.
20. The possibility of “going astray” in terms of one’s spiritual advance process.
I believe we can find more concepts if we dig and delve further, but even on the basis of the first twenty shlokas we can already see that the number of essentially new elements that had not existed prior to Patanjali and were introduced in his work is really big.
 In all fairness we should note that the Jungian model of advance as a process of individuation is unlike the rest of psychoanalysis based upon the model of "accumulation".
 The term is not a fluke. It is the emergence of the inner observer that in many schools is referred to as the "awakening".
 Here we should note that the entire metaphor of the cart connotes with the basic plot of Bhagavadgita: Arjuna riding his chariot between the two fighting parties, distracted between emotions and collisional requirements of Dharma. And he has to make his choice.