My knee-jerk reaction is horror. It looks like we've ended up with some emo hipster hybrid of that douche in Twilight and k.d. lang after a sex change. The new show-runner is Stephen Moffat, who crafts more adult and more effective Who stories than the previous king Russell T. Davies; but this kind of thing has happened before, where darker scripts are wedded to an inadequate actor with goofy hair, and it didn't turn out so well.
But then... I realized, as the title suggests, that I am old, and when I think "he needs a hair-cut" and "how can anyone so young be taken seriously as the Doctor", I must admit that's just my age talking. The truth is that we know nothing about this guy, as we knew nothing about Tennant, who turned out to be one of the best Doctors we've yet had. And my Doctor - the one who was there when I first discovered the show - is Peter Davison, who was not much older than this Smith character. So I will remain optimistic.
Though I still think Paterson Joseph would've been superb. While he would've probably played it differently, his Marquis de Carabas from Neverwhere would've made for an excellent interpretation in and of itself.