First, the Doctor should be played by a British actor, or someone who can put on a very good British accent (anywhere in Britain, just British in general). That is because it is a British show, and the Doctor is based in British locations whenever he ends up on Earth in the present day. So yeah, definitely a British actor.
Second, the Doctor must NOT be played by a woman. Time Lords and Time Ladies are seperate genders, obviously, so it would be completely against established shop canon if, as many have said should happen, a woman was to play the next Doctor.
Aside from those two basic rules, I don't give a stuff what the Doctor is, as long as the person playing him is a good actor. Matt Smith is a good actor, and brings a brilliant dynamic to the role. As as, and did, Dvid Tennant before him. And as did Christopher Eccleston, and all the other Doctors to date. As far as this thread is concerned, namely the casting of a black actor for the role, apparently Joseph Paterson was one of the actors in talks to take over the role before Matt Smith was chosen. And after seeing him in stuff like Survivors, I'm not surprised. The man is a very good actor. Another two people I'd consider are Chiwetel Ejiofor, and Colin Salmon.
But right now, I'm perfectly happy with Matt Smith. He's doing a cracking job, and with the changes Steven Moffat made when he took over for Series 5, the show has never been better (and I thought the older revived series episodes were amazing...). Personally, I hope things don't change from now. We have a winning formula, no need to change the mix so soon, eh?