In Short: Be nice. Be respectful. Consider the Platinum Rule – “Treat others in the way they like to be treated” (and I prefer that to the Golden Rule, because how others might like to be treated may *not* be what you would like).
Longish version: There is a difference between being “nice” and being “good.” And there is a place for both (typically I prefer good to nice, but in this space, I expect both. Saying “be nice” seems to sum that up).
Being respectful is not the same thing as respecting an opinion or a person. I’m not a language cop, and in my own personal spaces, I tolerate disagreement a little differently than I will accept it here, given the nature and purpose of this space. Here, I expect any and all commentary to be respectful, even (and especially) in disagreement. Disagreement does not need to be disagreeable. Adherence to the Platinum Rule is a very simple expectation (and no, trying to assert that another person enjoys being treated like dirt so that’s what you’re doing will not work here).
Finally, some things that seem to me to be common sense probably should be spelled out plainly. No racist, sexist, homophobic, transphobic, etc language will be tolerated (even if it’s framed “nicely”). No trolling allowed. These seem obvious to me…but I may be wrong. In the event that I am mistaken, I reserve the option to delete comments and to block repeat offenders.