Short, non-spoiler version: I enjoyed it! Funny, entertaining, and I thought it handled Loki well, a more emotionally complex role than in Avengers but without brushing any of his actions under the rug. All in all, a very fun film and a good follow-up to the first Thor movie.
As I say, I thought they pulled off a good balancing act in the writing for Loki: he definitely has some pretty mixed emotions going on towards his family members rather than pure hatred, but at the same time, he still doesn't place any value at all on anybody else's lives. (There's an interesting counterpoint in the conversation between Thor and Odin about how to handle Malekith, which has the effort of positioning the three of them as a trinity rather than just Thor and Loki as opposites: Thor wants to do anything to prevent lives being lost, Odin will sacrifice however many lives it takes for a principle, Loki will burn through lives as meaningless without a thought. For all that Loki refutes the father-son relationship, most of his worst qualities are an amplified version of Odin's.)
Thor and Loki on the dark elf ship together is the moment when I really got a sense of them as big and little brother, and how they must have interacted in happier times. Their battle with Malekith on the other world is interesting; it's immediately obvious that Loki's 'betrayal' is a pre-planned illusion, but the scene keeps you guessing as to how much else on top of that is also unreal. Loki's wound clearly isn't as fatal as it looks - but was he ever really even wounded at all? Was it all a planned machination, deliberately pushing Thor's emotional buttons, or was any of his rush to save Thor and dying apology genuine? All up for debate in a way that leaves a lot of options open, for both fic and canon.
Another fascinating scene that plays with illusions versus reality is the early scene of Loki talking to the projection of Frigga in his cell. She speaks some hard truths, keeping Loki's crimes in perspective, but at the same time it's never made explicit whether it's Frigga projecting the illusion to talk to him, or just Loki talking to one of his own. There are arguments for reading it in both ways - it seems unlikely for Loki to be capable of that much honesty with himself, but on the other hand Odin having stated that he'll never see her again and the way Loki banishes the illusion immediately after yelling that she's not his mother lend weight to the thought that perhaps she's his own projection. (Especially since neither of them is shown to cast illusions beyond their line of sight in any other scene.) Personally, I kind of like the thought that perhaps even Loki isn't fully sure. The later scene where we see the difference between how he really looks and what he projects to Thor is another nice moment demonstrating just how much he dwells in illusions.
I was glad to see Frigga have more of a presence in this film, even with it ending in her death. Killing a female character connected to the male leads is always going to have unfortunate overtones, but this didn't feel too much like a fridging to me, for two reasons: firstly, because she dies in defence of Jane and Asgard, fighting against an enemy who has no personal vendetta or particular interest in her male family members, so it's not a death that's about the men in her life at all. And secondly, while her death arguably acts as a motivating factor for Loki, that's handled in a more plot-driven, conscious way than just being about Loki's emotions: Thor risks going to Loki because he hopes that Loki will care about Frigga's death enough to help them, and while Loki does appear to grieve somewhat, it's questionable whether he's doing anything but going along to serve his own plans. So I guess it feels to me like it's more of an organic part of the plot than just a motivating plot device to make the male characters sad/angry. (A pretty fine distinction, I'll admit, but the fact that the film has multiple other female characters in varied roles also helps reduce the bad taste.)
I liked Darcy a lot in this; I can't quite remember what impression she made on me in the original Thor, after having read a ton of fic where her quirkiness was dialled up to the max to the point of being grating, but happily canon manages to balance things better and here she's funny without being over the top. Loved Jane being so much the inquisitive scientist all through the film as well.
I was less keen on what they did with Selvig, but then that kind of goofy slapsticky embarrassment humour isn't my cup of tea at the best of times, and at least he gets to do more than be comic relief by the end. Other than that I thought the humour was integrated pretty well; there were a lot of moments that had the audience laughing at my showing, without it undercutting the more dramatic scenes at all.
Glad to see Heimdall get more of a role, and even a little bit of action this time out. Sif got some nice moments, too, and I like that there was no real attempt to make any love triangle between her, Thor and Jane at all triangle-y; Odin thinks Sif is a better match than Jane, Sif seems to be into Thor, but there is no awful angsty pining or cat-fighting over it. Yay!
Misc other things:
* The casting/costuming for Bor in the backstory section at the beginning was fantastic. Looked like he'd just walked straight out of the comic book.
* Completely Gratuitous Shirtless Thor is not of any real interest to me, but I appreciate them throwing it in just on general principles.
* The unexpected Avenger cameo made me (and most of the audience) giggle. Much better idea than trying to wedge any of them into the plot more ham-handedly.
* Of course the convergence of all nine worlds occurs at Greenwich. Gloriously absurd comic book logic at its finest.
* Everybody at the windows, filming the fight on their phones because hey, look, it's Thor out there waving his hammer around: ha, yes. Both a nice nod to the Avengers as celebrities and a little bit of variation in the usual "passers-by are only there to run and scream" routine.
* Thor getting on the tube to get back to the fight made me smile a bit; the cut back to it a split second later when the train starts moving cracked me up. Somebody knows their tube travel.
* Post-credits scenes: the first one with the Collector was sufficiently left-field that I have to wonder what people not up on comics/upcoming movie details would have made of it. The second one at first seemed cute but hardly worth waiting through the credits for, but then the very final few seconds did make me grin.
Hey, look, I wrote an essay! So, yep. Enjoyed this one.