I'm sure this David Tennant guy will be fine, although I didn't watch any of Casanova, so I can't really comment myself. But it just annoys me that a whole incarnation has been used up on only one series, especially given the previous one was wasted on only a one-and-a-half-hour film. There are only 12 to play with, and the remainder need to be conserved! *mutters things about responsibility in the general direction of Christopher Ecclestone*
Also, if this must happen, I'd ideally like to see an older Doctor follow Ecclestone, just to keep a bit of variety in the role. Sure, there's a fine tradition of younger Doctors to follow - especially Peter Davidson. But I'd like to see an actor who can tap into those aspects of the Doctor's character so splendidly explored by people like Jon Pertwee.