I've gotten the impression from D3 that they just care way more about that IP than SC2. I mean, with SC2, they just repeatedly kept saying 'we're doing what we know worked in the past.' With Diablo, they're saying much of less of that. It seems like just about every system in Diablo 3 has gone through one or more iterations, from it's base in D2, to the prototype, to the revised prototype.
So it's going to be different, and I think people that were disappointed because SC2 was more of the same will have a reaction on the other side of the scale to D3.
For me, despite all the gamey stuff they're doing that I'm not so much a fan of, there's lots they're doing that I am.
1) Randomly generated dungeons with more-fixed overworld maps, and feature placement.
2) The Assault Beast, the Thousand Pounder. Seriously those fights all just look delicious and fun to play with friends.
3) The rune system is going to be tits. Lots of fun just adding runes to your character and seeing how they change your attacks.
4) More to do in dungeons besides just kill. The mini-quests, the environmental hazards, the mini-bosses.....
So yeah. I think there's going to be a lot to sink one's teeth into in D3 that's new and worth trying, as opposed to SC2, where many people feel like all that time and effort could have been better spent making a more interesting product.