There are many comments I find myself leaving often. Probably the most common is a reminder to back up answers, but "be nice", "one question per question", and "visit the help page" all come up frequently too.
On other stacks, I have observed something like standard comments that get left in these situations. They aren't formal, just nice copy that is being recycled.
I wanted to open up a space where we can develop our own common comments in the hopes of improving the quality of our interactions with users.
I will post my own below to get things started. Let's try to keep one "comment" per answer so users will know what they are upvoting/downvoting/suggesting edits too.