Spoilers!
The opening sequence was pretty awesome, particularly River diving off the skyscraper at the start (though I could tell how she would get out of it), and it really did look like Amy and Rory had been killed when Cecil L'ively appeared to turn on them all, especially with that Hannibal Lecter/Pandorica-type prison the Doctor was being put into. The best moment was the sudden reveal that the TARDIS has a swimming pool, which was then used to save River's life; it seems like the usual wacky stuff that Steven Moffat likes to write.
There were a few things that reminded me of older episodes - the characters recording messages for themselves was kind of like the one Amy did for herself in "The Beast Below", and the red flashing sensor that Amy was talking through made me think at first of the data ghosts, though it was a lot less creepy that either that or the Stone Angel using that dead guy's voicebox.
The climax was impressive, with an amazing fight scene at the end - in a spaceship that looked exactly like the one that Mulder got held in in Season 8 of The X-Files, but the most intriguing moments were at the end. First of all, there was the very sad moment when River had what was (for her) the last kiss with the Doctor, although inevitably, we'll see a lot more of those - probably ending in her slapping the Doctor because she doesn't know why he's doing it (I hope we won't get to the Doctor meeting River and her just saying: "Who are you?" too soon, though if Alex Kingston ever decides to stop playing River, then that could happen.
If she does want to keep playing River, though, I'd love to see some sort of spinoff series about her adventures when the Doctor isn't around!
I felt sorry for Rory when Amy poured out her heart to the Doctor (again); not surprisingly, even the Doctor was confused by this; the whole thing reminds me of the whole Cordelia/Angel/Groossalugg love triangle from Angel's third season - I'm hoping they don't end up the way Rose and Mickey did.
And then of course, there was the sudden unexpected regeneration at the end, and the question on whether Amy actually IS pregnant. So, could the little girl actually be The Doctor's daughter - maybe the same one who appeared in Series 4 (I know they came up with some other excuse for her being there, but it was a daft one that avoided the whole concept of the Doctor having sex, presumably because this is too much of a family show). I'm rather hoping that my initial theory that she would grow up to be River Song is wrong, since the fact that she is in a relationship with the Doctor would just be wrong, really wrong (she couldn't be River, since there is no indication that River is not human), but the other question is who the mother will turn out to be; I'm rather hoping this will turn out to be River, though Moffat seems to be trying to imply that the mother is somehow Amy (and she belongs with Rory); of course, there is also an upcoming episode called "The Doctor's Wife", which could also shed some light on this.
The other question is of course, why she killed the Doctor, and whether this was just some way to fake his death like The Master did - or if that really WAS the Doctor (the girl might somehow regenerate into the image of the Doctor, thus also undergoing a sex change). Of course, there is also the possibility that she could SOMEHOW end up being the Master!
I suspect we'll have to wait until Episode 7 to find out.