I've listened to the score a few times now and it's grown on me. However, I still feel it is incomplete. I think the score works as incidental music throughout the film, and even for certain cues. I still think the movie needed a more triumphant end theme though, even something similar to Captain America. If it just had that extra epic theme together with the rest of the score, I would've been happy.
The Avengers theme that Silvestri uses could've just functioned as a secondary theme. It kind of has that minorish sound like the Star Wars Imperial March. Imagine if that theme was the main theme for Star Wars. It wouldn't have been quite the same. Star Wars has both the Imperial March and the Main Titles, which gives the film two separate moods - one more heavy and dramatic, and one triumphant and soaring. Avengers only has the heavy and dramatic one. That's where I feel it is lacking.