Corrected entry: Thor's hammer has a slightly altered appearance in this film from how it was seen in Thor. Where before it had a mostly brown handle with a brown strap at the end, now the end of the shaft has a silver cap, with the brown strap connected to each side, and the handle is more prominently ringed by silver. While some may rationalize this by suggesting maybe Asgard redesigned it between movies, remember that this is an ancient weapon and the Asgard do not seem like the type of people who would alter their holy artifacts with superficial changes.
Vader47000
22nd Oct 2012
Correction: Saying that the Asgardians "do not seem like the type of people who would alter their holy artifacts with superficial changes" is just your opinion. Unless you can provide irrefutable evidence that neither Thor nor the Asgardians would alter his hammer, then it isn't a mistake. Especially when you take into account that the look of Thor's hammer has undergone dramatic changes before in the comics.
Phaneron ★