I get a kick out of watching Tennant's irreverent Doctor. I mean, here's this Time Lord fellow, who has been to the ends of the universe and back, through to different dimensions, from the beginnings to the ends and he's still gets excited about running into a bona fide werewolf.
Sure it's serious, but it's also amazingly new and exciting. Not to mention after dealing with the likes of the Slitheen, the Dalek and such, a werewolf must seem like a walk in the park.
By the way: Spoiler:
Impossible Planet was very, very good. Black holes, cryptic writings, the occasional apocolyptic remark ("an inverted self-extrapolating reflex of 6 to the 6th power every 6 seconds" or something like that...)? This is certainly one of the better episodes of this season and I can't wait to see the conclusion.
Whose voice was that? It sounded a lot like Christopher Lee, but I'm not all that certain.
Edit: Spoiler:
Turns out the voice of the beast is Gabriel Woolf.