When i see roleplaying, i tend to interpret it litterrally as role being the point.
On that angle, the sims seems more roleplaying as the emphasis is about the role of various characters and their social interractions.
Games like Diablo or Fallout 3 seems more like adventures games, in which you have various way to explore dungeons and kill monsters, but in the end, you always do the same, and haven't much the ability to play a role and get social interactions. Those aspects could become non existent, or so much in the sideline, that it seems irrelevant to use Roleplaying to qualify them. It's like saying that Fallout New Vegas is a card game. Indeed, you can play cards in that game, but that aspect is minor compared to killing monsters or talking to npcs.
But in the other hand, if Diablo, Fallout 3 or Deus-Ex aren't RPG, what genre name could be qualified to use for them ? Is it possible for the game industry to change the genre name, to become more accurate ?