Avengers, even when it first came out, I always felt it being vastly overrated. The main reason Avengers got the hype that it obtained was because it was people were waiting for Superheros franchises to meet in one large spectacle. And Wheedon succeeded in that. But aside from the decent chemistry of some (not all, because Hawkeye as well as Black Widow just didn't mesh) and the Hulk scene stealers and of course the after credit scenes, the movie as a whole was a total MESS. Everything about that movie makes my head hurt. I don't knock others love for the film, but it's not something I find nor have any desire to want to watch repeatedly.
GotG however, it wasn't perfect storywise due to Disney/Marvel's knack of not taking chances when it comes to storytelling, but it is without question the MOST entertaining Marvel film since the 1st Ironman, and may arguably be BETTER than Ironman. There's very little I can complain about the film, all the characters had chemistry, it was fucckin' hilarious, and it was almost self-aware not to be taken seriously. And THAT SOUNDTRACK