Matt Smith and Tom Baker vie for my favorite Doctor, but I am glad they are going with an older man again. We've had two younger Doctors and Nine was not-quite-older. We're due for a little more 'gravity' than the whimsy that Tennant and Smith brought (though, yes, please note I am not saying 9 and 10 weren't capable of being serious - but Smith especially is a lot more 'silly'/whimsical than the average Doctor).
Capaldi's been on Doctor Who before as a bit role, it looks like (2008, The Fires of Pompeii). I think I'm gonna see if we can re-watch that episode to see how he interacts with the Doctor/Donna.