After a quick glance, I think a lot of people speculate that it'll be something kind of like WoW's weapon skills. You get better at certain skills or talents as you use them more. This is all speculation, though, I guess. I haven't been following D3 since like the first few months after it was announced.
I don't know. I guess I can't say Diablo is identified by the whole talent tree / custom stats page, stuff, since Diablo 1 didn't have the tree, if I recall. I think Diablo 1 had spell books or something different - it's been too long, I can't remember. But I feel like if they go too radical with the talents it could kind of kill the who Diablo feel. If it were something dynamic that took away from you choosing your talents, it'd ruin the game for me. Most of the fun was in the fact that everyone's char could be slightly different based on their decisions in their talent tree and their stats. I just hope this is the same in D3.