Heh it's hard to think of what to say really because it's certainly not a bad episode, but it's just not a terribly exciting one to me when we're introduced to a group of new characters and have to start caring about them in a short space of time. I'm definitely in DW more for the character moments with the main cast, while characters are usually pretty thinly-sketched in the base under siege episodes, and this was no exception IMO. (Rather surprisingly actually considering that Toby Whitehouse generally has put a lot more emphasis on characterisation in his past episodes and tended to focus on the Doctor/companion relationship, although of course this did still have the odd moment of the Doctor worrying about Clara becoming too much like him)

Maybe it would make a difference if the horror was more affective for me, because as you say a lot of it is about horror and creating an atmosphere of claustrophobia, but none of it jumped out at me as particularly scary which probably didn't help.