I was also slightly reminded of The Thing. And that made me think all the way through that the humans were right, and the Doctor was wrong. Val, or whatever she was called, the horrible mother (are there any other kind for RTD - sorry) was a bit like Mrs Bennett in P&P: ridiculous in every way except for being ruthlessly right. Like Ash bringing John Hurt on board the Nostromo, you shouldn't let the alien through the air lock, even if it's fixed to your friend.
I need to quote calapine (hope you don't mind) "The terror, my very favourite kind: cheap and claustrophobic. Scenes so wonderfully long. I mean, it's a bloody stageplay and OH HOW I ADORE IT." Well said, it was very stagey, and David Tennant did an excellent job.
I'll probably think of more in a minute.