Sad news but hardly unexpected. In this day and age, there's no way an actor will stay in the role as long as Tom Baker did. Considering David "Biggest Who Fan" Tennant stayed in the role for four years, I didn't really expect Matt Smith to go much further.
As for the next actor play The Doctor, I know that I and everyone else here won't be able accurately guess who it will be (at the end of the day, it'll be someone most of us haven't heard of), but that won't stop me from speculating or dreaming.
These days I find myself hoping for the likes of Idris Elba and Ben Daniels (and not just because of the recurring rumors), but for old time's sake, I have to throw out Eddie Izzard and Alexander Siddig, too. Also, Hugh Laurie to Stephen Fry's The Master would be incredible. I would die and go to heaven if any of those fine actors were cast as The Twelfth Doctor.