After fours years its not really surprising, and heck, its longer then some doctors. Colin Baker and Peter Davidson only lasted for two years.... Its Tom Baker doing it for nearly 7 years that throws most people.
Anyway.. I liked Matt as the Doctor... Sure he wasn't David Tennant, but as pointed out, he was the Doctor when the show was becoming its most popular again, so of course people will remember him more, but Matt brought his own flavour to the role and made his Doctor truly his.... Think we can all agree though he was way better then Eccleston.
Anyway... as for the new Doctor? Much as I'd love to see Cumberbatch taking the role, think that with his Hollywood success he may not come back and do it.. be great if he did though, afterall he's naturally ginger I believe.
As for the other suggestions? A female timelord would cause way too much trouble with the fanboys. Yes, its progressive and all that... but 11 generations and never a gender change to suddenly get it? There would be too much kicking off and backlash... the same with the rumours about them replacing him with the first black doctor... again, would cause too much trouble with the fanboys and the BBC won't want that.
Bookies favourites are apparently Russell Tovey, who I don't like. He was only good in Being Human, he's been crap in everything else he's done. And Rupert Grint. I can see too many people making Harry Potter comparrisions if that happened so I just can't see that one myself.