My overall impression: Not a great movie, but a fun one. I was definitely not disappointed but i certainly wasn't overwhelmed either.
Part of the problem is that even though i tried to pay as little attention to the trailers as possible they still over-hyped the movie a little. If you've seen the trailers, just be warned that the movie may not be quite as epic as the trailers made it out to be (or at least not as epic as i was interpreting the trailers as making it out to be.)
Part of the "problem" is that this movie is following up on the Avengers, and it's hard to top everyone teaming up to fight an alien invasion. That problem was exacerbated by the trailers going "hey look! it's another alien invasion!" But then it turns out that the "alien invasion" is a single ship, and it's not actually an invasion it's more of a raiding party, and it only happens in the last 15 minutes or so of the movie.
And speaking of invasions/raiding parties, it seems pretty clear the the Asgardians are a race that's in decline. 5000 years ago they fought an entire army of dark elves to a standstill. But now a single dark elf ship is able to lay waste to their capital/home world/whatever at will. Yeah they had some help with the bit of sabotage, but even without that it seems would have had the upper hand. They would have just had to start bombarding the rest of the city until Odin, Thor, et al, were forced to come out and fight them.
When Natalie Portman first got infected with the ether i was really expecting her to get a super-power upgrade, a la Penny in Iron Man 3 so i was kind of disappointed when they didn't go that route.
I did appreciate that pretty much the first thing Portman did when Thor showed up again was to slap him, he pretty much deserved it. However i still don't feel that much investment in their relationship. Probably in part because i think both Sif and Portman's assistant are much cuter than she is.
And i've got to say that by this point they're going to have to go to pretty extraordinary lengths if they ever expect us to believe that Loki has actually been killed. Absolutely no one in our group was buying it this time. It is interesting though that Loki was able to pull of what he did without technically betraying Thor in the process. Not that anyone is really going to appreciate that nuance once they figure out what's happened. I'm presuming at this point that Odin isn't actually dead, just imprisoned somewhere. Though with Loki it's always hard to tell which way he'll go.
Oh yes, and as others have mentioned before, if you're watching a Marvel movie then you sit through ALL the credits. You don't just walk out after the mid-credits teaser.