Wow, I'm honestly a little surprised at how "meh" the reception has been for this confirmation. I mean, no one is saying he's a bad actor at all, quite the opposite in fact, but there's an awful lot of opinions being voiced that seem to feel he won't do the role justice in some way. I figured I'd throw my own two cents into the hat, and offer some friendly (repeat, friendly!) counter-arguments to a few of the points I've seen raised that I feel, personally, shouldn't give people so much reservation about the film.
1 - Accent. Yes, Dr. Strange is American...in the comics. The MCU has been pretty willing to tweak and outright change details to suit their own narrative, using the comics for much of their inspiration without being shackled. They might simply decide that this version of Strange is British, or not even really reference is accent too much. Or, Cumberbatch might simply put on an American accent - it would likely be better than people would be willing to give him credit for, because everyone has that deep British accent in their head when they see him, but hey, time shall tell.
2 - Too young for the role. Can't say I follow this one, I mean he looks young for his age, but it wouldn't take much for the makeup team to age him considerably if they want to go for that angle. Of course, this might also be a story that focuses a lot on how Dr. Strange was prior to acquiring his magical powers (I know, I'm hoping it's not an origin story either, but I feel given they're likely going to have to spend time introducing how magic works in the MCU, it's going to require at least some substantial flash-backing in order to connect Strange learning magic alongside the audience), so they might have needed an actor that could look convincing both younger and more aged. I'm assuming he's had enough time between learning magic and establishing himself as a presence, since he's referenced by name as a target of note in Captain America 2.
3 - Not "fun" enough. Again, not really sure why this is a factor for some people, but I will agree that there has certainly been a lot of enthusiasm shown by the actors for their roles thus far. I mean lets face it, they're getting a ton of recognition and money from films that even a decade or so before would have probably been the kind of embarrassing, for-the-money job that actors would sometimes need to do in order to fund themselves while they wait for the more "dignified" roles. However, Marvel has been putting some serious effort into overhauling just what it means to be a superhero movie - Guardians of the Galaxy was a superb action/comedy, and a love letter to the 70s/80s as a whole. Captain America 2 was a pretty good spy thriller. For Dr. Stange, they've gotten a director whose main credits are all horror movies. While they're still clearly enthused for the work they're making, Marvel also needs actors who can bring across the weight needed for their more serious work, and lets face it, Cumberbatch has a sense of gravitas about him that will likely lend a bit more credibility to people who think the MCU films are still "just" superhero movies.