If the film delivers critically, the audiences will roll in IMO. This isn't always the case, sure, but the GA already knows this will be an action packed spectacle. What they don't know is if it is worthwhile story-wise. If we can get solid performances, epic battles, and quality storytelling intertwined into one juicy summer film, than potential movie-goers will see those qualities through and pay for admission.
Nothing works better for summer audiences than a film with good action and a good story. It worked for Iron Man....
BUT
Hemsworth needs to SELL Thor, like Downey sold Tony Stark. The word of mouth for RDJ's performance as Iron Man was what really killed it.