I like the concept of alternate HP
I would
love an alternate magic system, but I think that would stray as much from the "core" notion of D&D as mass combat would
I never use psionics and avoid the Planes entirely, but I am always surprised to see how many people really, really like them
D&D, given its general direction, is fine as it is. It is very, very far from the Be All And End All of fantasy games, but it serves its over-the-top, nearly superhero vision of fantasy pretty well. That fits with a lot of fantasy literature out there.
Personally, I prefer "realistic fantasy", a la
Lions of Al-Rassan and the
Earthsea books -- the stories may be big, but the people are more down to earth, normal, and common. The magic is something that happens subtley or in the background, never just because it can be done. Does D&D fit this style of play? No, not really, although it can be forced to fit it after a fashion. In the end to alter the core rulebooks to any substantial level would lose core fans and would add very little to the experience. Players like myself who A) prefer another style of play and B) are used to tinkering with rules can still have our fun by cherry-picking what we do and do not like.
So, it's all good
