Hmm. Well, let's see if I can add a bit to this. Might take a little bit, though....
I've spoken a number of times about what a martial art actually is (not it's techniques, but it's underlying, or guiding philosophy), and how that reality of what a martial art is is really what makes one art different, or unique, when compared to another, particularly when there is a great deal of similarity in the arts themselves (superficially). And I think that a number of people here have gotten a fair bit of what I mean by that. However, there is still a degree of "old" thinking colouring this description. So I'll try again, and explain one aspect a little more than I have in other posts.
A martial art is not it's techniques, or it's training methods, or it's weapons, or it's ranges, or it's rule-sets, or anything similar. A martial art is an expression of an underlying, or guiding philosophy. A philosophy is really just a collection of beliefs and values which hold an internal consitency, or congruency, and are interlinked together. These beliefs and values then get expressed in a physical form, for martial arts taking the form of combative techniques and training methodologies. Psychologically speaking, these would be the "behaviours" of the "personality" of the art (the underlying philosophy giving decision making approaches to differing situations).
Now, I've spoken about how this philosophy can be social, cultural, economic, personal, spiritual etc, lending to the methods of the art itself, and it seems that most take that to mean the techniques. And while that is true, it's only part of it. The other aspect that is highly dependant on the philosophy is it's training methodologies themselves. For Koryu, that means a structured kata-based training method, with the techniques not changing in order to maintain the lessons the way they are intended to be transmitted, rather than allow the personality of a particular instructor "colour" the art and the lessons it has to teach. For competitive arts, that means a high emphasis on testing in a competitive arena, and training methods geared towards that (conditioning drills, sparring etc).
When it comes to an art such as Wing Chun, while all arts are based in the concepts that structure the systems themselves, they demonstrate and transmit them in different ways (Koryu kata transmitting the concepts by maintaining the methods that best encapsulate them, for example, which may be seen as being based more in "technique", although that really is missing why the training is the way it is). Wing Chun, due to it's guiding philosophy, has it's training methodologies dictated by that philosophy, and that philosophy says that the art is taught by constantly new expressions of it's concepts.
Now, another thing that may be confusing some is actually the very idea of these "concepts". I think a number of people are mistaking "concepts" with "strategies and tactics". For Wing Chun, the strategy (overall plan) is to survive/succeed/overwhelm with the concepts of the art. The tactics (methods used to achieve the strategy) include the physical techniques, such as Pak Sao, Bong Sau, Lap Sao, chain punching etc. These are not concepts, they are tactics. The concepts give rise to them, but they are not the concepts themselves.
So to say that Wing Chun teaches by concepts rather than techniques, making it unique, is completely accurate. Other arts certainly have concepts unique to them within their teachings, and concepts that help govern what those arts do, but the teaching methodologies (rising from the underlying philosophy) of these other arts are very different to the conceptual base of Wing Chun.
As to the body positioning, leverage etc in Wing Chun, well, that's again just in the way they train and teach the lessons they have to offer. But I don't think anyone can state that they are the only art that uses leverage to win.... although I have heard many arts claim that that is what sets them apart (using knowledge of leverage over strength, so a smaller, weaker person can triumph over a bigger, stronger one.... hmm, don't think I'd want to train an art that required you to be bigger and stronger in order to win! But for the record, there is a self defence method that stresses size and strength over anything else. It's called body building), such as BJJ, Judo, Aikido, Ninjutsu, and many more. It's not unique, but the methods used to express these ideas, teach them, and train them are.