I'm convinced that Marvel execs decided they needed to put out this movie because so much of the Marvel media-verse has gotten dark. Avengers, Thor, Captain America, Agents of Shield, etc all have dark and somewhat downer storylines of betrayal, death, and generally doom-and-gloom.
Guardians has none of that, even though Thanos is nominally a villain of the piece. This is a movie about five misfits getting together to kick some serious ass, have a lot of laughs along the way, and generally get you to shout "YEAH! That was AWESOME!" a lot. Which you do, if you are me. Also, you say "I am Groot" more than a few times, and laugh about that as well.
The scriptwriters are not trying to be deep or meaningful, which generally works but left me feeling like all the characters' stories got shortchanged. You get only a few inches deep into Peter Quill's story because it's needed to make the movie ending work, and not even that deep into the other characters' stories. If the movie has a failing, it's this. Say what you will about Joss Whedon's writing, but he has shown over and over again that it's possible to have an ensemble cast and tell coherent stories around, and with, them sharing spotlight time. The writers for GotG do a passable job, but just barely.
The place where this sticks out worst is Gamora-Nebula. Clearly there's a lot of back-story and history that could have been brought out. Much of that story is key to how these women relate to each other, and what motivates their choices in this film. But other than a few almost throw-away lines of dialog you really don't get a feel for it.
On the plus side, what really stands out for me is how the CGI characters work. You have a tri-syllabic living plant-thing, and a hyped-up gene-modified mechanized raccoon with a machine gun who's the brains of the outfit and you never see them as CGI creations. They are full, living, participating characters. The last time I think this was done so well was Farscape, which used muppet-style puppet characters as major players and made it work. Here the other actors and the film's editors really sell you on Groot and Rocket. I never once got pushed out of my enjoyment by realizing they were CGI.
Finally, a nod to Michael Rooker as Yondu whose character has morphed away from what I think of as GotG canon into something that feels very much like an homage to the Dread Pirate Roberts. This is a character I wanted to dislike and ended up loving because he epitomizes the "Mad Max meets Loony Tunes" feel that carries a good chunk of the movie. Yondu also spearheads a lot of what I think is fan service in this film. It will mostly go over the kids' heads but for the adults who grew up in the 70s and 80s and weaned on that music and media there's a constant stream of humor and nostalgia. Plus a great selection of music.