A relief, really. A few newspaper reviews painted it as merely competent, not a 'perfect' film. Like you said, even that might have been enough to avoid ruining Avengers by association (as in the kind of pall Green Lantern might cast on a Justice League movie).
If there was one that could've been screwed up royally it would be Cap's 'big nice guy patriot' persona in an age where such a thing has been all but relegated to the dustbin of history for Superheroes. Ironically, a lot of the storylines taking place after he was thawed out play on that kind of conflict- Cap being too uppity and morally straightforward for our time. The same personal problem as with Superman really, but for at least twenty years of movies, TV and comics he's had to deal with the way his powers and the propaganda of his chief nemesis work to distance him from his adopted people despite all his efforts. Cap's not quite that strong, but such a tension between him and the regular soldiers he works with is certainly feasible. He has to earn their respect through teamwork, to prove himself more than just a mascot in bright colors.
Hopefully Marvel can continue this winning streak into the main event, bringing out the best of these comic book legends when they're together. As Fantastic 4 proved, group movies are even harder to do right.