Thor: The Dark World opens up with information overload.
We learn that thousands of years ago a weapon called the Aether was used by beings known as Dark Elves to send the universe into darkness. The Asgardians stop them and the Aether ends up being hidden not to be discovered for thousands of years. Meanwhile Thor has been fighting for peace in the Nine Realms while his lover Jane is stuck dating Roy.
This amount of exposition is pretty exhausting and sets the stage for the downfall of the film.