I haven't played D&D since high school. I've never been able to get over certain objections I have to the system. I can't even take the system seriously any more.
First: Alignments. Especially alignment languages. Blah.
Second: Weapon restrictions. Clerics can't use daggers? Magic-users can't use swords? WTF? Same with armour restrictions.
Third: The hit-point system. People should not, in my opinion, have more hit points than elephants have. I get the story about what hit points "represent". I think that the mechanic is barfy.
Fourth: I'm generally down on the idea of character classes in general, but I can accept the concept. But why do non-humans have level limits? Hell, I think that the whole level system is kinda b0rken, but I can wrap my head around it. I can't wrap my head around the idea that non-humans are never going to be as good as humans at various character classes.
Fifth: Spells that you forget when you use them. Dumbest. Thing. Evar.