“Thor’s new sleeveless look was to achieve a less-armored look for the conversational scenes between the Avengers. The look also takes a cue from the comics and works well with Chris Hemsworth’s incredible physique. His muscular arms are the iconic statement of Thor, and the discs and red cape are the iconic statements of the cotsume.”
— Costume designer Alexandra Byrne
I just had to include the quote about Hemsworth’s “incredible physique”.
As always, if you like this stuff you should seriously consider buying the book because it’s awesome.