Captain America: The First Avenger (Plus the Other Avengers)!

First of all, Captain America was GREAT! Marvel is just killing it! I was actually not very impressed with Iron Man the first time I saw it, but I've rewatched it a couple of times and I've come around. RDJ just does an excellent job with the role. I'm still rather apathetic about the big battle sequence in it, though, but overall, it's pretty good. I completely enjoyed Thor, but I think I liked Captain America even more! I'm already a sucker for things set in the 1940s. Chris Evans does an EXCELLENT job embodying Steve Rogers' altruism, patriotism and just overall all-consuming passion for justice. You really believe him!
The special effect of making him scrawny in the beginning is just mind-blowing!
I must admit, there are gaps in logic and points where I was like "That doesn't make any sense" but I was willing to let it go, because overall, I just enjoyed the movie that much. (I think Thor was the same way.) Even moreso than Thor or Iron Man, you got the sense of tying the entire Avengers universe together. Howard Stark, Tony's... dad? Grandfather? Grandfather would probably make more sense, but I think it's his dad. But anyway, Howard Stark plays a supporting role in the movie and there's a cameo appearance of the original robot Human Torch in a scene at the beginning.
I liked that Bucky was reinterpreted from being a kid sidekick into Cap's best friend. In fact, in the beginning, Steve is more Bucky's sidekick. Also, SPOILERS, Bucky's death is reworked to give it more drama and I think it works much better than the last-minute comic book death.
I wasn't expecting the movie to involve Cap's awakening in the present (taken heavily from The Ultimates) nor the previews of The Avengers movie. THAT is going to be awesome!

It just worked. I think Marvel has tapped into the fact that for the most part, people want their super hero movies fun. They don't want the psychoses and neurotics of 80s comics. They want popcorn and that's what Marvel delivers.
At ComiCon, Marvel gave out four posters to fans featuring the characters as they will appear in the upcoming movie. Cap's costume has been simplified a bit from his movie costume. (My source.)
The utility straps are gone. I actually liked the fact that the stripes on his torso were redone that way. It was a fun nod, but made it seem more useful. The brown elements are converted to blue and red. I liked the brown actually, but this is more faithful to the comics. Overall, the new suit is more bright and comic book-y, so I can't argue with that. Some people don't like the painted on wings on the cowl, but I do. They're faithful to the original design, but not silly. Also, they're hardly even that noticeable.
Iron Man just looks like Iron Man. I couldn't tell you what, if any changes had been made.
Hawkeye's costume is basically his suit from The Ultimates. It's kinda funny that with Cap and Thor, they sort of made their suits LESS realistic and more cartoony and bright, but with Hawkeye, he looks more utilitarian. I can't quite make out what COLORS are used, but they are muted and dark. I wish they'd worked in some purple in homage to the regular comic book costume. I mean, they can skip the loincloth and buccaneer boots, but a matching color scheme would be nice. I'm completely alright with them not covering up Jeremy Renner's adorable face, though.
Black Widow's costume is fine. That's really all I got.
They didn't do a Thor poster, which is odd, since I think my right hand more people would have wanted that than a Black Widow or Hawkeye poster. But anyway, in Thor's movie, his costume was very much like a suit of armor and looked heavily based on the costume he wears now in the comics.
In the Avengers preview clip though, he has a short sleeve shirt, seemingly more closely based on his original classic look. I mean, why does an indestructible Thunder God need armored sleeves? Show off them guns, baby! (No cape, though... now that I think about it, I think Ultimate Thor has no sleeves and no cape, so maybe that's the real inspiration.)
Exciting! I really can't wait to see this movie! Joss Whedon, heed the wise advice of scholar RuPaul: "Don't fuck it up!"