In Diablo II, there is no role playing elements whatsoever. You pick a character and are forced in the story without a care in the world about choices. In a way, you simply follow the story like an adventure game. Yes, you level your character... But having a leveling system doesn't automatically make the game a RPG.
This game have a solid story and there is more adventure elements in this game than role play elements. Yet, nobody calls Diablo II an adventure game. Why? Because it wasn't written on the box, because adventure games don't sell as well. RPG was written on the box however and, even if it is false, people caught on to it because of its popularity.
That is silly.