Though he's not directing, Deadline reports that the decision between Branagh and Marvel was "mutual and amicable," with the Shakespearean actor and director likely to serve as a producer on the sequel. Disney and Marvel are no doubt lining up candidates to take Branagh's place, but given Marvel's history of choosing unlikely directors for their properties, it's hard to even guess who could be on that list. Nonetheless, feel free to speculate away.
We've known since even before Thor's release that a potential direct sequel was in the works, with one of the first film's writers, Don Payne, working on preliminary ideas for a storyline.
The July release date makes Thor 2 the second movie sequel for the summer of 2013, following Iron Man 3 on May 3, 2013. That film will be the first Marvel release after The Avengers, and Robert Downey Jr will be back in the goatee and armor under the direction of Shane Black, the Lethal Weapon writer who is taking the franchise reigns from Jon Favreau. Favreau directed the first two Iron Man films but opted instead to direct Disney's Magic Kingdom.
Previous Marvel productions, starting with Iron Man and going through this summer's Captain America: The First Avenger, have been distributed by Paramount Pictures, but from The Avengers on out, Disney will distribute the cinematic adventures of Marvel's mighty heroes.
Chris Hemsworth is the only Thor castmember officially confirmed to return, but it's likely we'll be seeing Anthony Hopkins, Ray Stevenson, Jaimie Alexander, and Idris Elba reprising their Asgardian characters. Natalie Portman and the others playing humans the first time around could also be back, particularly considering the nature of Thor's ending. Tom Hiddleston will definitely appear as Loki in The Avengers, but a continued presence beyond that is unknown.