Wow, just got back from Guardians. This is just my gut impression, it may change, but I think it may be my favorite Marvel film so far. Hope I'm not getting people's hopes too high when I say that.
This movie, in essence, feels like two things.
First, it feels as if James Gunn went through the whole internet and wrote a list of all the complaints people have with the Marvel movies - they don't have enough emotion, the tone is the same, too many setups, no good villains - and then said "Alright, let's see what we can do here." I got really emotional at several points throughout the film. Not counting Logan, it's the most emotional experience I had with a comic book film since Xavier's arc in DOFP and the ending to TASM2.
Second, it's Age of Ultron done right. Whedon initially planned for it to be "smaller" and more character-driven, but it eventually evolved into a rehash of the first film (not in terms of story, but you know what I mean). This doesn't. It has the charm of the first one (even moreso IMO), but also a tighter script - as if they just knew they had to rely on a lot more this time.
Really hope it holds up on a second viewing. It would break my heart if it doesn't.