Spoilers, of course.
I think the problem was that I felt absolutely no tension throughout the entire episode. I suppose it didn't help that I saw Moffat say that this was going to be a super-scary episode; I've found that whenever Moffat claims something about something coming up, it usually means that it's not at all that way but he's trying to hype it up. But sure, there are monsters and we don't know what they want and the Doctor's trying to figure it out - no better or worse than any other episode, really.
I think the thing I really disliked here was the Doctor's monologue about the four words, "dark, sword, forsaken, temple", because though it was meant to show just how much of a genius he is, there was really no logical way for him to figure it out. How in the world does he know that the sword refers to the sword in Orion's belt, or that the temple refers to the church in the town? He doesn't figure something out - he just pulls it out of thin air.
There's also the problem that the science is bad: if we can see the three stars lined up to make the sword up in the sky, we are not the fourth point in that line. (Not to mention that we might see three stars making a line in the 2-D sky, but there's very little chance that those three stars are actually lined up in 3-D space.) Doctor Who really needs a scientific advisor on staff like Star Trek has. I realize that DW is meant to be sciency-wiency and I am perfectly happy waving away their almost magical technology and technobabble, but when they call out to real-world science and they're completely wrong, it ruins the suspension of disbelief - if they're going to get it wrong, it's actually better for them to throw the real science out the window and make stuff up. This was one of the main problems with "Kill the Moon" and "In the Forest of the Night", and it took a while for me to warm back up to this episode after that. (I used to rant about the horrible science at the end of "The Poison Sky", but now it looks good in comparison.)
Loved the note cards. That was a universal hit, I think.
The sealab cast was pretty vanilla except for the leader (who seemed to have been given a handicap only so that there could be someone who could read lips) and
I wasn't expecting a two-part episode, but it sounds like it's going to be pretty common this season, and that's cool. A piece of advice to the writers, though: killing off the main characters does not make a good cliffhanger. The audience knows better. And doing it twice in a row only makes us mock you.