Peggy at Biology in Science Fiction put up a great post about one of the other reasons I'm disappointed to see Tennant replaced--the replacement himself.
Some of the rumors were that the eleventh Doctor might be played by a black actor such as Chiwetel Ejiofor or Paterson Joseph (who would have been my pick) or a woman like Alex Kingston or Catherine Tate. It doesn't take more than a glance at Smith's photo to realize that he is neither a woman, nor black. Maybe he'll be the greatest Doctor ever, but even if he is, I'm still a bit disappointed that he's a super young white dude.
Peggy's post digs into the history of Doctor Who to show that there's no reason that a Time Lord can't regenerate with a different gender or skin color. It's just stodgy conservatism on the part of the producers. They could have deliberately set out to make a more interesting choice, something that would have changed the dynamics of the show in interesting ways, but they didn't.