The spell list. Or specifically, having magic be a bunch of blocked rule texts that's separate from other forms of resolution. Any sort of skill-based or freeform magic makes the game into "not quite D&D".
Classes, and especially classes in a stretched out table with level, special bonuses, and class features in columns. When you monkey with that, you're in Heartbreaker territory.
6 stats, Str Dex Con Int Wis Cha. The order doesn't matter, but the stats do.
d20 based combat. Damage that varies in the dice used based on weapon type or the spell cast.