Despite being the main villain (and even being on the poster), Christopher Eccleston's name doesn't appear in most of the promotional material, including said poster.
The end credits sequence includes individual credits with headshots of most of the cast, including Alice Krige (who only shows up in one scene) and Chris O'Dowd (who appears in two) — but not Jonathan Howard, who played the much bigger and more plot-significant role of Ian.
Alan Taylor is not particularly fond of his experience on making the movie. He claimed that he was given a lot of freedom when he was filming only for Executive Meddling to change everything in post-production.
Idris Elba apparently didn't have a good time making the movie, and claimed the shoot was tedious and exhausting.
Dyeing for Your Art: Chris Hemsworth grew out his hair for over a year to have more authentic long hair rather than use a wig. (a particularly funny part of the Hilarious Outtakes is his hair entering Hemsworth and Natalie Portman's mouth during a kiss)
Executive Meddling: Malekith had more of a backstory to tell but because of Tom Hiddleson's popularity, Loki received an expanded role in the plot, which resulted in this backstory being removed to save time.
Hemsworth, Alexander and Hopkins from the last movie.
Zachary Levi (Fandral) is American.
Genius Bonus: Loki's Bond One-Liner to Kurse ("See you in Hel, monster."): it might not really be a reference to the more common Stock Phrase "see you in hell," but to the similar-sounding Hel (place for the dead in Norse mythology), ruled also by a person named Hel, who of all people is Loki's daughter in the original mythos.
Svartalfheim literally means "home of the black elves" in old Norse.
"Game of Thors", in reference to Director Alan Taylor's work on Game of Thrones.
"Black Hole Grenades" for the Dark Elf bombs that open vortexes to suck people in.
Fake Shemp: Natalie Portman was unavailable for reshoots of the ending kissing scene. The director had Chris Hemsworth's wife, actress Elsa Pataky, stand in as Jane instead, wearing Jane's costume and a wig.
Money, Dear Boy: Christopher Eccleston openly admits he did this one for the paycheck, aside from being able to collaborate with Alan Taylor.
The Other Darrin: Zachary Levi replaces Joshua Dallas as Fandral due to Dallas' commitment to Once Upon a Time. Ironically, Levi was supposed to play Fandral in the first Thor but had to back out because he was committed to Chuck at the time.
Throw It In: The gag of Thor hanging his hammer on a coat hook in Jane's apartment was an improvised bit by Chris Hemsworth who obviously thought that a warrior prince/god like Thor would be mindful of the proper etiquette of checking one's weapons when you enter another's home.