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Reminder to senior researchers to talk to your junior peers about rejection. 
Yes you worked super hard on your submission and it stings to get rejected. But the whole point of peer review is to get feedback from other researchers; usually, but not always, experts on the topics of your research
Most, but not all, feedback from reviewers is valuable. Sometimes the feedback is explicit: <Some paper you didn't cite> has a relevant result. Did you think about this other perspective? How about a different baseline/control experiment?
But even the vague feedback is useful.
"Paper was hard to follow" = A group of your peer experts couldn't read the result that they are expert in; maybe you can improve the writing and presentation.
"Paper was hard to follow" = A group of your peer experts couldn't read the result that they are expert in; maybe you can improve the writing and presentation.
"Paper repeats known claims" = did you miss a published result? did you fail to present the specific context in which your work differs from other, more famous results?
But some reviews are just bad.
"Result is trivial" is usually either a warning that something is seriously wrong with your work, or the reviewer has fallen for the fallacy that all new work must be complicated. It may be worth appealing to the editor/program committee
"Result is trivial" is usually either a warning that something is seriously wrong with your work, or the reviewer has fallen for the fallacy that all new work must be complicated. It may be worth appealing to the editor/program committee
Another common red flag is "citation fishing": claiming that you failed to reference a bunch of papers that, upon inspection, have little relevance and are suspiciously have one or more common authors among the papers
"You didn't write the paper I wanted to read" is tricky. Sometimes it's a complaint you can do nothing about. But sometimes it points to alternative ways to present your work.
Above all, remember that a rejection is an opportunity to improve, and that reviewers are human and fallible to egotism, narrow-mindedness and other foibles. But even the reviews that upset you at first reading often point indirectly to different, sometimes better, presentation
