When the basic plot of your first movie around a certain character revolves around him being a jerk and then changing himself, it can be difficult to make a second movie where you're literally just killing time until the next main Avengers movie. At least with Iron Man 2, they actually had some universe-building to do. But in this movie? Um... they introduce another Infinity Stone. That's about it.
This second Thor movie centers around a rare occurrence in the cosmos called the Convergence, in which the Nine Realms align perfectly and the lines between reality are extremely blurred... but only in one specific location on any given realm. And there's this supposedly extinct race of bad guys called the Dark Elves led by Malekith who wants to use this ancient vague weapon called the Aether and use the Convergence to destroy the universe.
As you can see, there's a fair share of sci-fi mumbo jumbo here and a pretty generic villain plot. Thor's continued story is kind of interesting, even if there's nothing major really happening in it like the last movie. But honestly this film probably has one of the weaker foundations of any Marvel movie.
Malekith, despite being played by Christopher Eccleston, is about as flat a villain as there is in the MCU. There's not really any backstory to him that makes him interesting, so he's basically just another dude who wants to destroy the world. As a matter of fact, it ends up being Loki who steals the spotlight--not only from Malekith, but from pretty much the whole doggone movie. We get to see much more of what the trickster is capable of in this film, and he is full of surprises. Tom Hiddleston just seems to be getting better in better in the role.
Luckily, Loki's not the only good thing about this movie. The cast is still pretty stellar. Despite a rather average plot, the film is still pretty exciting and fast-paced overall. There's a couple of pretty great action sequences, namely the Dark Elves' assault on Asgard. The climax is a bit of a confusing mess, but at least there's comedy in it. And there's comedy thrown in elsewhere too. Even if it isn't the most well-written of the Marvel films, most people who've enjoyed the MCU to this point shouldn't have much issue finding some fun and excitement in it.
Thor: The Dark World may be near the bottom of the MCU food chain, but it's still a quite passable couple of hours. It's another one of those films that's just good enough to help kill time until the next bigger Marvel movie. It definitely could've been a lot better, but I've said before that when even the worst entries in your movie series are well above average, you're doing something right.