It is in many ways impressive to think that Marvel has managed to get a Thor franchise off the ground and make it stick. Thor as a character is a tough nut to crack, founded on a pretty heady combination of cosmic mysticism and Norse mythology, neither of which are particularly tuned into the head spaces of your average film-goer. It's an uphill battle for sure, but the Avengers are legion, and Thor: The Dark World hits the theaters with quite a bit of momentum. With the origin story out of the way and the characters well established, the second film is a perfect opportunity to hit the ground running.
Unfortunately, this second Thor film starts with a bit of a stumble. One of the major drawbacks of a mythology like Thor's is you don't really have the opportunity to shortcut your exposition by drawing on the shared experience of modern society. If you contrast that with, say, Captain America, who spent his first film foray fighting Nazis (and everyone hates Nazis), you have a lot more work to do to get things moving. Marvel went with Malekith (Christopher Eccleston, though it's not like you'd be able to tell) and the Dark Elves as the central villain, and has to devote much of its first act to establishing why the audience should care. Because of this, the opening of Thor: The Dark World is a mess. We do have Chris Hemsworth to pull us through; he feels natural in the role and proffers a warm charm to the viewer to ensure they don't drown in the expository goings-on. The intrigue works marginally better back on Earth, as Portman and Kat Dennings' characters continue to provide a modicum of comic relief to contrast with the crazy stuff going on in the stars (though it is a shade disappointing that when Portman does travel to Asgard, she does get relegated straight into the damsel-in-distress role, which seems like a waste).
There is a point when the exposition stops and the action moves to the fore, when director Alan Taylor gets to settle in and just have some fun. His best decision is to take advantage of Tim Hiddleston (supposedly involving at least some amount of reshoots late in the game to expand his role), whose Loki is one of the most enjoyable characters in any of the eight Marvel Studios films that have been released. He's funny (and gets all the best lines), which draws you in, but the layers underneath his aristocratic humor base are what keeps you watching. Hiddleston juggles Loki's clashing motivations (the desire to rule, pent up hatred for Thor and Odin, his complicated parentage, a lingering love for Asgard he can't quite quell) with deft aplomb, and the screen sings when he finds himself in a reluctant team-up with Thor for a good chunk of acts two and three, culminating in a ripping action sequence based around invisible portals teleporting combatants from world to world and location to location that might be one of the best superhero action climaxes we've seen in years.
Hopkins' Odin is becoming a bit of a disaster on the second go around though. I enjoyed his arch Shakespearean turn in the first film (not too surprising there, considering Kenneth Branagh was at the helm), but he turns it up to about 15 here, randomly shouting lines, chewing scenery and vaguely making a mockery of the world. His face and his cache do bring along a sense of the regal, but that soon dissolves when his temper explodes for half sentences. He needs to be reined in before stumbling into parody. The last scene seems to indicate that he might serve a lesser role going forward, which is probably for the best for everyone involved.
It's difficult to tell whether it's better to start strong and end weak (like Iron Man 3 and The Wolverine did over the summer) or to start weak and end strong, like we have here. Taylor and screenwriters Christopher Yost, Christopher Markus and Stephen McFeely run the risk of alienating their audience with their long-in-the-tooth exposition scenes and general mishandling of the supporting cast (The Warriors Three and Lady Sif are basically window dressing, and Stellen Skarsgard's Erik Selvig is played a bit too much for laughs as he tries to recover from the trauma of The Avengers), but they end the film on such a high note that it's tough not to leave the theater with a smile on your face. This is not a great movie by any means; the film's first hour makes sure of this. Everyone at Marvel Studios has a lot of work to do if they really want to elevate these films beyond decent fan service. Thor: The Dark World isn't a complete step in the right directions for the spin-off films, but it is proof that they have created some strong characters capable of carrying the audience through bouts of mediocrity. A little more elbow grease and we might get something special sooner rather than later.