DominicCobb said:

If I have one complaint about his Peter Quill, it's that we don't get enough of him. There's definitely a lot about him that hasn't been said (how does he feel about being abducted? does he want to go back to Earth? etc.), and presumably will be in a sequel. But it really should have been said here.

The main purpose of this film was to establish the characters, organizations and settings of this world.

If they had explored those things about Quill in this film it would have been rushed. Leaving it to a sequel gives them an opportunity to explore it in depth in a story that has the potential to be more conducive to it.

I'm sure stuff like the scene with the Collector made comic fans wet, but, to me, it was just really weird and rather random and not entirely necessary.

I'm not familiar with the source material either but I think it's pretty apparent they're setting up the Collector to be a bigger character later on. It was already established in Thor: The Dark World that he posses one of the other infinity stones. And Benicio Del Toro is a big name actor that you don't waste on an unimportant side character.

This film is setting up a lot of things that will inevitably pay off in later sequels like Jon C. Reilly and Glenn Close's characters.

Anyway, the Collector scene was necessary from a storytelling perspective. First it allows for a breather in between action scenes and second it provides vital exposition that informs the main characters about what it is they're dealing with.