I don't think it was a romance at all. Neither in intention nor execution. Yes, there was some mutual attraction there, as is often the case when two single attractive people are thrown together by circumstance. But hardly a romance. Despite the attraction, in the end, the important aspect of their relationship was that they were teammates, not possible/probable lovers. I thought the Gamora/Star-Lord relationship was really well illustrated by Gunn, with restraint and nuance. I really see it as a strength rather than a flaw.
I too thought the Gamora/Nebula relationship could have been afforded a little more screen time, but considering where we meet Gamora, i.e. after she went rogue on Thanos' twisted little family, I don't think the opportunity was there. Maybe we could've gotten a flashback scene... but even that is tricky in that it could easily derail the forward momentum of the movie. All things considered, I give Gunn a pass on this as well.
I disagree about the Star-Lord/mom and Star-Lord/Yondu relationships as well. On the former, I don't think spending more time on Star-Lord's life as a child have added much. Neither in terms of pushing the plot forward nor in terms of emotional resonance. The connection Star-Lord feels to his mom literally resonates throughout the movie. I have similar feelings on the latter as well. The Yondu/Star-Lord relationship was pitch perfect IMO. Yondu's crew and especially Yondu himself are basically evil, selfish, cut-throat, classic villains. Yet, we see the inkling of begrudging tenderness and pride that lurks beneath in his interactions with Star-Lord... wonderful shorthand for the years spent raising, caring for, and more importantly, not eating Star-Lord.
It's that tenderness and pride that makes someone who, for all practical purposes, is one evil SOB, perform actions that are absolutely heroic in the end. Even in his last scene, when he figures out that he's been duped by Star-Lord with the ol' troll-doll-switcheroo, even though he
ought to be livid with rage, it's his pride in Peter Quill that shines through as his primary emotion. I thought it was wonderful.