Getting a paper rejection from a journal is always frustrating. So much time, effort and care goes into a paper that to have anonymous strangers say its no good can't help but hurt. I've just had a paper rejected, and it still hurts even though they had nothing bad to say about the paper.
The starfish paper I wrote with my students documents something that hasn't been documented before, but is certainly not the world's most important paper. It doesn't fit neatly into any field or derive from the pressing questions in any literature. So we sent it to a journal that explicitly says they don't care if it is important, so long as it is original, technically sound research. The reviewers agree that is passes these hurdles, but question its importance to their field. On this basis alone, the academic editor rejected it. I can't say I'm entirely surprised, as PLOS ONE has become a relatively high-impact journal. That type of success naturally brings them to function like a more traditional print journal competing for the flashiest papers. I filled out their feedback form to suggest that they update their stated criteria for acceptance, but won't otherwise protest.
There are two upsides to all this. The reviewers found the paper convincing and novel, with no technical or language faults. So we just need reformat for another journal and submit it there. Perhaps more importantly, this gives my student co-authors a window into yet another aspect of the scientific process that most of their peers never see.