Diablo 2 hardly has the most innovative of play mechanics though, does it? Most of the compulsion comes from the collecting and trading of items rather than any outstanding gameplay features. Obviously there's some strategy and skill involved, particularly in PvP, but it basically boils down to clicking on stuff lots, especially for melée characters.
I'm not dissing D2 at all - I find it very compelling. I like the character-building, the community aspects, the free online play, the lure of finding new super-rare items - but I would never suggest that it's doing anything particularly clever or difficult or innovative.
So what I'm really trying to say is that I don't think there's any need to fear for D3 just because a few guys have left Blizzard, because I know there are a thousand things I could think of to make Diablo better, and I'm not even a games designer.