GOTG more or less remade its characters wholesale, and sometimes I'm cool with that (Drax, Groot, Nebula) and sometimes I'm not (Gamora, Ronan). I'm ambivalent on Star-Lord, never quite sure how I feel about his portrayal.
MCU Thanos kinda sucks - when you strip away the death-worshipping he's a pretty bland character.
Don't really like what they ended up doing with Agatha Harkness; I wouldn't call myself a huge fan of the comic version, but going from "creepy old witch lady who's surprisingly 100% good" to "bad witch" is a shame.
A lot of Thor's supporting cast and villains were wasted, but I'm not sure if that's necessarily because of poor depiction or throwing away the chance to grow into their role. Hoping Natalie Portman's Jane is better under Waititi, at least.