See, that's what I love about Tennant: he plays funny so well, yet when he needs to be serious, he's serious, i.e. "No second chances; I'm
that sort of a man." and, as fresnelly pointed out, the genius "Don't you think she looks tired?"
The Christmas special really had a lot of GREAT lines for The Doctor IMO, both serious and comedic. Almost makes me feel bad that most Americans will have to wait over a year to see David Tennant in the role ;) :D
EDIT: Wow, Wikipedia really has a TON of info on Doctor Who:
http://en.wikipedia.org/wiki/Portal:Doctor_Who
http://en.wikipedia.org/wiki/History_of_Doctor_Who
http://en.wikipedia.org/wiki/Doctor_%28Doctor_Who%29
http://en.wikipedia.org/wiki/Doctor_Who_theme_music
And much more!