Well, to fall back on the old cliches, European cinema tends to be thought of as more self-conciously arty, more concerned with visuals and ideas and less concerned with plot and narrative. An easy shorthand for me is that Alien (directed by a Brit, creatures designed by a Swiss guy), is, in a lot of ways, a European film. Impeccably designed, beautifully made, filled with haunting imagery, but with a simple plot, thin characters, minimal dialogue, etc.
Whereas Aliens, though made by a Canadian, is a very American film -- tons of memorable characters and memorable dialogue, but much more fast-paced, action-packed, story driven, and much less about mood, suspense and ideas than its predecessor.
Whedon is a huge fan of Cameron and Aliens, and he fits into that lineage to me, as does his film -- he even said Cameron is who he studied for the action scenes. So my feeling is if European critics are embracing a very "American" film like Avengers, American critics might like it even more. Make sense, or am I reaching?