As for modification possibilities, Carmack stated early on that doom3 would be _very_ mod friendly, why else do you think all the tools for it are to be included with the game? (even in a nameless leak you could find some of these tools if you knew where to look) Besides you're missing one big point with doom3.
Im judging this only from the various trailers I've seen and from id software themselves.
The Animations: Any other game I've seen don't quite come close to the animations of doom3, personally I think Carmacks goal with doom3 is to take gaming a step closer to CG Movies (a'la shrek, toy-story, final fantasy, etc) when I watched the doom3 trailer I was really amazed by the cinematic quality of the game. As a matter of fact id stated themselves that they used motion capture for the 'human' actors ingame.
In the end its all about what tickles your fancy, do you prefer baseball or soccer? I know im gonna buy both games and play both games and I'd love to see Counter-Strike 2 on the source engine