I wasn't really grabbed by this episode until later on--the whole "we have to rush, rush, rush!" kept me from having time to be emotionally involved, I guess. There were guest characters, but we didn't get a lot of time for them, so I didn't care about them nearly as much as I could have. The computer guy was the only one who got a little development, and the captain, though I felt like she had less, as she had a definite role to play, and the role showed more than her personal attributes.
I did like that David Tennant got to scream quite a lot. He did that very well. Also the writhing.
I don't think the Doctor has ever admitted to being scared before. Correct me if I'm wrong. I'm sure he has been scared, just not admitted it. That was the most interesting part of the episode, for me.
Yay, Martha gets a key! Yay, the Doctor didn't go all emo, except for a moment. Unless the screaming counts as emo, which I don't think it does.