Honestly, I can't say anything about the film any better than anyone else has said. Nor can I revert to supurlatives to describe how good it is, as many others have done ("Awesome" being the descriptor of choice)
As others have pointed out Mark Ruffalo makes Bruce Banner interesting and compelling. He's a quiet, calm guy who gets across the idea that he is holding back a beast for the first half of the film without showing the actual transformation. That's the mark of a great actor.
It's was also clever storytelling: having everyone talk about the Hulk (even showing Black Widow afraid of Banner) before showing the transformation.
The Hulk himself is brought to life by Ruffalo (through motion capture) and a great swollen team of effects guys. Nevertheless, the Hulk is believable and even likable.
Chris Evans is plays Captain America as the straight-arrow he is but still makes him interesting. Same goes for Chris Hemsworth as Thor. Great stuff.
Robert Downy Junior as Tony Stark can do no wrong.
Scarlett Johannson gives Black Widow depth and a guarded vulnerability. She also has a fabulous ass.
Scarlett Johannson's ass in 3D. "nuff said.
Jeremy Renner is great, but his transition from Loki-controlled bad guy to being accepted back into the team happened way too fast and with hardly any angst or second-guessing. Weak.
Cobie Smulders as Maria Hill. She's hot, she's from my hometown of Vancouver and she kicks ass like nobody's business. I look forward to seeing more of her, either in The Avengers 2 (or as Wonder Woman if Joss Whedon could work that out).
Nick Fury jumping out of the crashing helicopter? Uh-uh. Nope. I know Samuel L Jackson is a badass but I would believe Captain America or Thor doing that, but not Nick Fury.
Agent Coulson. At the end of the day, he was part of the team.
Loved all the Joss Whedon touches. No one writes or directs an ensemble like Whedon. His dialogue is crisp and snappy and never devolves into cliched non-sequitors. The through line of the story never gets lost.
My favourite line: "That's my secret. I'm always angry." Means nothing out of context but in the context of the story it is amazing.
I've been here and there. I've drawn a lot of pictures. I've written a bit, too. I'm not good at this self-promotion thing. Look, you want to know about me? just visit these websites. Okay?