i've only played AD&D 2nd edition and D&D 3.0 and 3.5
AD&D 2nd: It felt like what, at the time, I considered to be an RPG. Classes were not just "what you could do mechnically," but were the actual niche you filled. A Fighter fights, a wizard casts spells. You don't see fighters who sneak around like you can do in 3.5 with cross-class skills. Multiclassing and dual-classing aside, you are what your class is. I liked this. It was like playing the old Final Fantasy games--trite, but with tried and true archetypes that are readily identifiable. The problem? As time progressed, players wanted versatility. The system began to buckle. In order to incorporate fighters than could sneak, The Complete Ninja Handbook came out. To make unarmed fighters and pugilists, some kooky rules were developed for martial arts--that just. didn't. work. In short, AD&D tried to be more than what it was designed to be by adding a new table with every new product. Eventually, the system collapsed.
D&D 3.0: This system was designed to appeal to audiences who wanted the versatility AD&D was never designed for. Problem? It's still class-based. If you want versatility, you don't use a sytem that relies on sharp distinctions. The good point was that Prestige Classes added a new layer of strategic play; the downside is that it introduced a new level of number crunching. The price of "versatility."
D&D 3.5: Some of the problems of 3.0 were fixed; some were carried over (such as core rulebook feats being comparatively weak to the Complete series). A lot of "homebrew" styled material from 3.0 as well as in-play bugs were smoothed over, everything streamlined. However, 3.5 is even less niche-based than 3.0; I daresay it completely lacks the niche-flavored style of AD&D since multiclassing is so integral to survival as a melee fighter and, in some cases, as a caster. The feat system is starting to degrade due to the power incline introduced by the Complete series.