I know when a lot of people see this, they automatic jump to "Thor is just happy that Cap can't lift Mjolnir!" But honestly, I think that the way his face drops, he's disappointed at the fact that Cap can't lift Mjolnir.<<<<<<<<< THIS COMMENT THOUGH!
Look at these amazing actors. Look at the actors' eyes and then at the characters. (Might as well ignore the top; I don't know why they gave Tony Stark two pictures of himself but they're the same.) The actors look playful, but then when in character they are suddenly so changed.....look at Captain America - so starkly different from the fun loving Chris. LOOK AT TOM SMIRKING