As the Marvel Universe has expanded it's become clear that there are some things that it does really well and other things it falters at. They've done a great job of casting, with every role being played an actor who knows how to ground a character who has a ridiculous power, but they rely too much on Macguffins to power their plots, creating a sameness between all of their movies that has contributed to superhero fatigue. They've created an open ended world for their characters to inhabit which is cool, but a few times they've fallen back on generic villains like the dark elves from Thor 2 or the lame lava-handed guy from Iron Man 3. You could sum it up by saying that they do a good job of making modern blockbusters, but the modern blockbuster giveth and it taketh away.
Guardians of the Galaxy, then, is very much a Marvel movie. The highs of the movie are really high, but there's some weaker spots, too. They do a great job of introducing us to the five main characters in a way that gives them all distinct personalities, but then they also overload the movie by throwing in too many villains. I get that they want to establish Thanos for later movies, but he doesn't have any real impact on the plot here, so every time he appears it's a distraction in an already crowded movie. Indiana Jones could just defeat the Nazis, he didn't have to defeat the super-Nazis that were the power behind the Nazis.
The characterization is also a bit uneven. Rocket Raccoon has lived a hard enough life that his handful of sad moments work well even though most of the time he's being sarcastically funny, and Groot, the killer tree with a gentle heart, seemed oddly plausible because he is mostly used as a protector for his friends and not as a roving bad-ass. But on the flipside, the human character at the center of the story was a bit iffy for me. Peter Quill is meant to be a devil-may-care rogue, but there were times when his jokes didn't make sense given the seriousness of the scene he was in. If you think about the great rogues in movie history, they might crack a lot of jokes, but they are serious when the stakes are life and death and they need to sell the drama of the scene. (John McClane might taunt Hans Gruber when he's watching him fall off the side of the skyscraper, but he's just cursing when he's running across glass with no shoes on.) Quill's acting like he was in a comedy would have made more sense if he didn't already have so many sidekicks there to be the comic relief and if the plot didn't concern the end of the universe.
That said, at the end of the day Guardians of the Galaxy really does do a good job of revitalizing a genre that was becoming increasingly tedious. It has a lot more imagination than a lot of the more recent superhero movies - yes, the ending does have a spaceship crashing into a planet which seems to be de rigeur now, but I can forgive it that because all of the alien worlds we saw before then were more original. And most importantly it's a lot more fun than a lot of the more recent Marvel movies which were straining for a seriousness that they just couldn't achieve. (Cough cough winter soldier.) You have to look at it like a numbers game, and if one of the villains is a bit unnecessary and one of the heroes is a little tone deaf you still have a few other villains which work well and four other heroes that do exactly what they're supposed to. Honestly, I like those odds.