Thinking about it, the tonal shifts didn't really bother me. They seemed natural for the most part.
Ronan was one-note and uninteresting, but he was serviceable in moving the plot forward. In other words, he was your typical Marvel villain.
What did bother me a little was how quickly the ensemble went from hating each other to risking everything for one another. The Kevin Bacon reference was funny, but I didn't feel like that romantic scene between Gamora and Starlord was earned whatsoever.
And this is just a dumb nitpick, but the Collector's "aid" was awful. I... will... no... longer... be... your... slave!!!!