Those ideas get re-used because they're good! Why should each game have to reinvent the RPG system? Personally, I much prefer Divine Divinity over Diablo 2 because it has more depth.
But why to copy all the ideas from one game? And by the way, Diablo series has quite a depth if you just read the manual's story and books scattered around Diablo 1. Did you know that Diablo isn't the only super unique that you're fighting in both games? Warlord of Blood (D1) and Bartuc the Bloody are the same person/demon...
I think I see what's happening here. This is a guess, so tell me if I'm wrong. You have a tendancy to use stronger words than necessary, giving a different impression. (I do that too. <img src="/ubbthreads/images/graemlins/shame.gif" alt="" />) By "rip-off", you mean that it's similar, but you don't mean that Larian deliberately tried to remake Diablo under a different name or produce no original content. Is that right?
I'm just wondering that why Divinity looks and feels quite much like Diablo...