1. I am, I freely confess, a bit sad. The Wilderness Years have spoiled me more than a little; when a Doctor from the Classic Series leaves the part, it means they do audios instead of videos and we get a couple dozen paperback books about them. When an actor from the New Series leaves, we don't see them anymore. Even for the 50th Anniversary Special, we're only getting two out of three. (Or three out of four, as it turns out.) Things like this make me miss the Missing Adventures even more.
2. That said, it's not too surprising--I was actually a tiny bit surprised to hear that we were getting Smith for Season Eight. Three years has been a pretty standard tenure for the part, with only two actors doing more and two doing less (and one of them involuntarily.) Three years is long enough to show everyone what you can do, let them get used to you, but not so long that they get sick of your mannerisms and feel like the show should end with you. (I do think that Tom Baker stayed too long, to the detriment of the series, despite loving the stories he did while staying too long.)
3. And I think Moffat has one more Doctor in him, but no more. It's a working theory of mine that any showrunner can come up with an idea for the Doctor by emphasizing what hasn't been emphasized in the previous Doctor's tenure...so Davison was vulnerable and young to counter Baker's omnipotent smugness, and Tennant was confident to contrast with Eccleston's survivor guilt. But trying to reverse it again leads to bad results...Colin Baker went against Davison, but in trying to not just make him another Tom, JNT emphasized exactly the wrong things. (McCoy, I think, was mostly a creation of Andrew Cartmel. And he agrees.) So if Moffat stays more than another two to three seasons, I think we're in trouble.
4. I was hoping that we'd get Smith's replacement mid-season this time, at the next Christmas special. But I suppose there are contractual reasons for that. (There might actually be some contractual reasons for his departure, period. Everything up until now has been pointing to him staying for a fourth season. But we'll probably never know.)
5. I really think that the various "who will it be?" speculations are all going to be wrong. Everyone tends to pick "hot" actors right now, especially people that Moffat has just been working with (so this time it's Benedict Cumberbatch, last time it was Darren Nesbitt.) But those people don't want a part like the Doctor; they've just been elevated to the point of big-name Hollywood actor by their previous roles, they're not going to take a part that primarily serves the career purpose of getting you in front of a large audience and letting you show your range. They've done that.
6. So we'll get a relative unknown (as much as I'd like to see them cast a non-white male in the role, I suspect it won't happen under Moffat) and another year or two of Moffat, at least. And frankly, I'm happy about the change. One constant about Doctor Who is that it's always a good time for a change, even if you've been entertained by what's gone before. Exeunt Matt Smith, come back for the Big Finish audios when the license gets rewritten to allow it, and roll on Doctor Twelve. As a lifelong Who fan, I wouldn't have it any other way.