For me, the following are required to make it D&D:
Dungeoncrawling. That should always be the main focus of the game. There can be other things to do as well, but D&D isn't D&D if you don't spend most of your time crawling through a dungeon(or temple, or forest, or whatever) killing monsters.
Class and level system. Point buy really doesn't suit D&D. Being a level 10 fighter really means something and is far more evocative to me than some guy that has 10 points in the attack skill and calls himself a fighter.
Party composition is important. The default assumption that the average party is fighter, rogue, mage, cleric is a big part of D&D to me. Now, the fighter doesn't neccessarily have to be a fighter. The classes could all be variants of some sort(eg, druid instead of cleric), but the basic party structure has alot of appeal to me.
The twenty sided die. Optimally this should be used for most task resolution, especially involving life and death. I'm sure there's better ways of doing it(a d20 is a very random thing, after all), but it isn't D&D unless you're using a twenty sided die.
Hit points. Sure, they are unrealistic as heck. Sure, there are probably better ways of handling injury. But that wouldn't be D&D. I like my epic dragonslaying heroes to be able to view the local town guard with utter contempt. If a high level character is going to die, it should be from a dragon or beholder, not from a peasant with a knife or from falling damage. D&D isn't a simulation of reality. It's a game about going into dungeons and killing the dragons therein.
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