Diablo is very different from Starcraft.

I think if people are just after a multiplayer-focused RTS that Blizzard is still capable of delivering a top quality experience.
Where I am skeptical is their ability to deliver a top quality single player campaign. SC2’s campaign had some of the worst gaming writing I have ever seen — and I’ve seen a lot of bad writing in games. The missions themselves felt over-designed / QA’d to the point of just being tutorials for individual units.

They can do the multiplayer, I’m confident they could pull that off. I also don’t have any interest in that. Can they make the single player experience fun? I’m skeptical, but would like to be proven wrong.