Hiya!
Niche-based Classes, Levels, HP, AC and the base idea of getting a group of "un-normal" people together to go into a deadly and dangerous dungeon to kill monsters and take their stuff.
That's "D&D" at is base bedrock level. To me anyway.
You can mess with all of that and it will still be "D&D", but boiled down to "if we remove...", then removing any of those will, imnsho, stop it from being "D&D".
As an aside...and I'm sure this will be a bit controversial to some...I also believe that the idea of "DM as Adversary" is a D&D thing. A DM, IMHO, is someone who provides
challenges in his world that the players PC's can/will face. A DM is *not* someone who provides
fun in his world as defined as "situations where the PC's can be heroes and win". Somewhere along the line (mid through 2e if I had to guess) the game's idea that a DM was supposed to provide "challenges that the PC's should be able to overcome" became the mantra. This mutated into the abomination that is now "The DM is there to help the players have fun". There is a difference between the two ideas. The first, the more "old skool" idea of the DM providing
challenges is that overcoming said challenges
is FUN. The second, the more "new skool" idea of the DM providing
"fun" is that
unless the PC's overcome those challenges, the game is somehow "not fun".
Anyway, the point of that aside is that "DM as Adversary", to me, is one of the bedrocks that makes D&D, well, D&D. Removing that "I'm going to try and kill all of your PC's...but I'm going to do it fairly and honestly" and replacing it with "I should always say Yes and never kill off a PC unless absolutely unavoidable" has done a terrible disservice to the game's history.
Then again, I'm a "killer DM" when looked at from today's standpoint, so there is that...
^_^
Paul L. Ming