It occurred to me today that there are quite a few things that we, as human beings, are naturally inclined to do on SO but really "ought" not to for the sake of the site. They're the kind of things that we rely on the documentation, the culture, personal discipline, the incentive systems, user feedback and in some cases moderator action to keep our "basic tendencies" in check.
For most of these things, the system "works", but for some, human nature is collectively getting the better of us. By identifying those things in one place, perhaps we'll be better positioned to address them.
(Note: I realize there are some SO behaviors that are "controversial" in the sense that there is no consensus that they are "bad". I think listing them here is ok, since it's still helpful to be aware of the human tendency.)
Update #1: Not sure if this will sufficiently address the concerns of the "primarily-opinion based" close voters, but answers should point to metrics where possible (e.g. slope of close vote review queue size).
Update #2: For those of you who consider MSO as an extension of SO, this question is about SO exclusive of MSO.