2011's Thor was in my humble opinion a nice popcorn film, nothing complicated but also nothing grievously offensive to people who are familiar with the comic book hero. It was perhaps the weakest of the new Marvel films next to all bar 2008's "The Incredible Hulk", but still a better film than many of the non Marvel superhero flicks. Thor did one thing very well, it established Hemsworth as a likable and charismatic lead which he has carried through "The Avengers" and now "Thor: The Dark World". It also introduced us to Tom Hiddleston's Loki, but that character didn't really take off until "The Avengers" imho, when Hiddleston seemed to really start having fun with the part.
Thor: The Darkworld is an odd film in that it has exactly the opposite problem to pretty much every FX blockbuster of the now. The standard pattern for the majority of the big Genre films in recent years has been a great first two acts followed by a mediocre or just plain bad third. Hollywood is so obsessed with twists, subverting expectations and sequels that so many films rocket off to a great start and have no idea what to do with themselves after throwing CG at your head for 60 minutes.
Thor: The Darkworld has the opposite issue, a weirdly plotted and put together first act that gets much much better after Natalie Portman gets injected with space goo. The primary issue is that the first act does a very bad job explaining what the film is about, and tries to seed macguffins and character points in an awkward manner that doesn't really work as a story, more as a series of connected events. This I think has led to many reviewers to criticize the plot as being a bit daft and hard to decipher despite it actually being relativity straight forward. It centers around an old foe of Asgard; Malekith lord of the Dark elves who fought Odin's father in an attempt to return the universe to the primal darkness that existed before creation. He utilized a weapon; the Aether, that was capable of destroying worlds but was stopped and the Aether was trapped (because as a force of pure destruction it could not be destroyed). Malekith was thought dead but really escaped with a small cabal of his followers awaiting a time where the Nine realms converge to release the Aether so he could try again. Now he is back and the Aether (space goo) has possessed Jane Foster and our hero has to fight against the returned Malekith, deal with his imprisoned brother Loki and the objections of his own father to save Asgard and the whole universe.
Once the plot establishment is out of the way the film brings out some nice action scenes and decent character development for our mains despite quite a large cast of secondary characters and a number of location shifts. The final action sequence in particular is great as it almost seems like a parody of FX fueled action sequences, with the convergence causing reality to bend and character teleporting between locations and worlds as they fight. (It almost plays up how nonsensical so many FX sequences have become now days as well as the fundamental unreality of film time)
The Dark elves are a reasonable villain, Malekith is played well by Christopher Eccelston but the script makes him a very straight line antagonist outshone like the rest of the film by Hiddleson as Loki. They are very well realized visually, I love the creepy doll masks and the overall knife blade look of all their tech. Sadly I feel they are made to appear potent by making the Asgardians appear weak, especially Odin who ends up being wrong about absolutely everything in this film; which is weird seeing the whole first film was essentially an exercise by Odin in teaching Thor wisdom and humility. (Although this plan seemed to make a villain of Loki and destroyed the Bifrost and part of a New Mexico town) The other character disappointment is Siff who seems to be leading towards a love triangle plot with Thor and Jane but is then basically not used after the escape from Asgard. I don't mind the Brave companions not being used in this film, or being used minimally, but something was started in the first act (the scene at the feast in particular) which basically has zero payoff later on. It goes back to the first act issue I mentioned above, it's almost like a different set of writers wrote the first act. Basically its a bit of sloppy writing, the only named female Asgardian who isn't his mother HAS to be in love with Thor, which is fine except is seems to have no affect on the plot other than placing a tiny bit of doubt on her helping Thor escape. Which she does anyway, so what's the point? Use those scenes to explain the plot more gooderer. The tension with his father more than enough demonstrates Thor being trapped between two worlds without having to make the only female hero with actual superpowers in the current Marvel film universe (well bar the brief Pepper Potts thing in Ironman 3) faun all over him and basically nullify herself as a character useful for plot by then NOT DOING ANYTHING WITH HER. (Sorry long rambling sentence but it bugged me.)
But overall it is like the first Thor film a good solid Comic book film, true (bar Odin) to the characters and feel of the other Marvel films. Probably a little better than the first as the plot and situations are just a little more epic, fitting with the power level of the character, and the universe the first film only touched on is fleshed out and seems more substantial in this film.