This was a pleasant surprise of a movie, full of great humor as well as an intriguing story. It's quick and zany at times. Think Psych in space (back when Psych was funny). The pace is quick and the story line is easy to pick up and follow.
What makes the film work is something that early Lucas and Joss Whedon understand: great sci-fi movies are really westerns at heart. There is a relationship with nature, a broken, compromised hero, a deep necessity for teamwork, and the story happens OUT there in the wild. Guardians of the Galaxy has ALL of this.
Guardians is already set up for a sequel so we will see if Marvel can continue the magic. It's a great addition to the Marvel universe.