Intrinsically good, but in conflict with some larger, out of band concern that the contributor could have no way to know about? Upvote to take the sting out of rejection, along with a note along the lines of "Well done, and we would merge is it weren't for our commitment to support xxx systems which are not compatible with yyy. Perhaps refactor as a plugin?"

Also, upvotes and merge decisions may well come from different people, who happen to disagree. This is in fact healthy sometimes.