You're right, it did work pretty well for me and I've read none of the comics and seen neither Hulk film, nor Thor, nor Captain America. All I knew was from the two Iron Man movies and I thought Avengers worked gangbusters. I didn't even think Thor's introduction was awkward, I loved how brute force and unshowy it was, although I at least had the prior knowledge of his relationship to Loki.
I think your point about leaving room to up the ante for sequels extends beyond Loki to the way the fight was contained to Manhattan too. They were saving the Earth, but only slugging it out in NYC. Next film will have a megabudget to match the first's megabox office so they're bound to at least be able to afford to spread out to New Jersey. Maybe even Schenectady.
I never knew ABBA was a supergroup though. I mean, I knew they were super, I just didn't know they were separate somebodies before they teamed up to fight one of their Scandanavian gods.