Umm, for those discussing the engine. You do realise, that the Unreal engine still exists from Unreal 1? It's just up to a ridiculous build number now. I'm sure the engine has probably been completely rewritten a few times by now, but Epic develop the Unreal engine, then use that to make games that will encourage more titles using that same engine. Which of course = more $$$ for Epic.
And Wingznut, good point about D3, I've always thought that id's development on that game was weird. Ever notice that everytime another breaking game is going to come out/announces new features/whatever, Doom3 gets pushed back and a month or so later they announce groundbreaking technology which these other games already have?