Strip out any economy. Gathering gold – typically a staple of D&D-based RPGs – is outta there.
Strip out NPC interaction outside of combat. Or any idea of “quests” (which hadn’t quite gelled as a regular feature of RPGs at the time anyway).
Story is paper-thin, without much more plotline or background than your run-of-the-mill roguelike. […]
Though people who claim Diablo isn’t an RPG might be on thin ice maintaining that this game series is, in spite of it being based on the original dice-and-paper role-playing game rule system.