Navagon: Diablo on the other hand, I don't even see where the attempt is being made to include RPG gameplay. Your character is nothing but a killing machine. The NPCs are nothing more than quest givers and traders and the world is nothing more than a place where you do exactly what is asked of you or you don't get to progress. It takes some of the mechanics, but makes an entirely different kind of game with them. Much like the upcoming C&C game using the Battlefield 3 engine. It doesn't make the game an FPS.
Your summary sums up most RPG games - even many table top ones. NPCs that you interact with typically are quest givers or traders - or just generic people that give a stock 5-10 dismissive or short reply answers with no depth to them at all.
Most RPG games focus around killing as primary quest goals - its not the only result, but a major part of many of the games is killing monsters and badguys (and good guys sometimes).
And in most games if you don't do the quests you world and game does not advance nor does the game.
I think you're being more harsh than you intend to carry your point across and its failing, because based on your assessment Diablo 2 has little different to something like Icewind Dale or Baldurs Gate.
As said by others the title RPG has a wide range of subgenres underneath it; some are diehard RPGs where combat is minimal and its a political game (mostly tabletop); Some are hack and slash like Diablo 2 or Torchlight; others are "traditional" like Baldur's Gate; some are JRPGs; and even LARPing fits under RPG.
As for the last comparison I've no idea what you mean there; the game engine is just the graphics engine used to display a game and has nothing what so ever to do with the content that the designers decide to develop using that engine. Heck the Starcraft 2 engine and editor can be used to make an RPG game if you wanted or even a hack and slash or shooter RPG instead of a strategy game - even though they are using the same game engine.