What is D&D? According to some people on the ‘net, it’s their favourite edition, and anything that comes after it is no longer D&D.
I like 4e, so every edition of D&D is D&D to me.
However, there’s a couple of basic core concepts that I believe that D&D has to have to remain D&D:
Attacking in melee: You roll a d20, higher is better. You compare to a target number which is calculated based on the “AC” of the monster and on your class/level. There may be other modifiers. If you hit, you roll polyhedral dice to determine your damage. This is subtracted from the target’s “hit points”, and at 0 hit points, the target dies (or is knocked unconscious).
Class/Levels: You have a class and a level in that class. As you gain levels, you gain more abilities and power. Your accuracy improves, etc.
Everything else is window dressing. Every edition has had different takes on what you add to those core concepts. Look at the vast number of 3e classes that try very different things: Incarnate, Binder, Warblade, Psion. Are they all D&D? Surely so – but they have very different takes on how things work.
The difference between 3e’s Warblade and a Fighter in 4e is there, but you can see one as the first attempt of the next. D&D is an evolving game which supports a wide range of mechanics. Thief skills per AD&D weren’t even in the original release… they came later. From that original flat percentage system, we’ve now reached the 4e system of d20+modifiers vs. target, which shows a lot of resemblance to the attack system. (I wonder why? 
)
)