Things I enjoyed:

Each of the characters was well formed with their personality and humor. The scene where Quill is putting together a plan and everybody is bouncing off each other -- perfect example of how this film works.

Rocket's drunken scene was surprising in its poignancy.

The usage of music in this film was unbelievable and inspired. Using 70s pop music was fantastic -- the music was a character.

I thought the usage and intro of Thanos was very well handled. He already seems like the most interesting foe we've had since Loki.

Many, many great things in the film.

Quibbles I had with the film:

There was a surprising amount of exposition where the characters are telling each other how much they like each other. The whole appeal of this group is that they are a reluctant team that eventually come to like each other. This part was rushed and really happened too soon.

John C. Reilly didn't fit. Stunt casting.

The pacing is probably my biggest complaint. The film had a lot going on but the pacing was really off. It skipped some parts of the plot -- like better establishing who these characters are -- and slowed down to a crawl in other parts -- the scene where Groot covers up everybody takes eons.

The bad guy was a waste of Ronin, a more complex character than this film offered. Ronin in the comics is not malevolent, just unbendingly ideological -- he's teamed up with heroes many times and could have been a valuable warrior against Thanos. Instead, he's a one-dimensional baddie just like the dark elf in Thor 2 in every significant way.