I'm going to open with the caveat that I don't read comic books, I just watch superhero movies, so there will be no comparison to the source material here. For me, this film is the culmination of the worldbuilding Marvel Studios has been doing since the first Iron Man movie, and that world stands on its own.
That said, I really enjoyed the movie.
The plot was serviceable - no frills, no surprises - but in a movie whose premise is getting together a bunch of big-name characters, this is to be expected. Each of our heroes gets their moments of awesome, and the main storyline is just there to string them all together - which it does just is fine.
But boy, what moments of awesome there were. I had a hard time deciding which of the main character was my favorite, though there were definitely some - Iron Man, Thor, Loki, Captain America, Agent Coulson - who shone brighter than others - Hawkeye, Black Widow, Hulk, Nick Fury.
Iron Man had most of the best lines, and RDJ's performance was flawless as always. Some of the best scenes were when Thor and Loki went one-on-one (I'm a sucker for family drama). Agent Coulson has always been a great supporting character, so his genre-savvy heroic sacrifice really packed a punch. And Captain America was just perfect in every way imaginable.
He also brought on, for me, the biggest moment of fangirling in the whole 143-minute fangirling spectacular when he responded to Black Widow's comment that Thor and Loki were "basically gods" by saying that "There's only one God, and I'm pretty sure he doesn't dress like that." (Yes, a superhero professing belief in monotheism is the sort of thing I fangirl over.)
I only have two real complaints. The first is that, in what could have been a very character-driven film, the character arcs were all rather compressed (although Iron Man's was very good). This is to be expected, though, as this is still a superhero movie and we do need lots of good fight scenes and explosions and such. I suppose I can't even really complain about this, given that it's merely a function of the genre. And the fight scenes were very good.
My other problem was that Loki, who was such a great sympathetic villain in Thor, came across as a bit inept - at times comically so - because each of the Avengers had to get their chance to one-up him. The beat-down given to him by Hulk was particularly egregious. Though funny, it would have been better suited for a villain the audience actually disliked and wanted to see get taken down a peg.
Overall, while perhaps not quite as good as some of its predecessors in the Marvel Live-Action Universe, The Avengers was good entertainment. I walked away satisfied, and while I'm not rushing out to buy another movie ticket, I look forward to watching it again on DVD.