It’s not just side conversations that do it for me. It really irritates me when someone—who usually didn’t read the details—misinterprets the question or answers the wrong question, not what was asked, and then the other responders take their cue from her and answer the same wrong question.
It also bugs me when someone enters the thread just to criticize the question, or worse, the asker. Silence is always an option. If you don’t like the subject matter or the point of view, just leave quietly.
But for me the biggest nuisance is the ones who seize every opportunity to post snide and mocking remarks, almost as if they thought they were funny, and to pound the same old tired wisecracks. I get that that’s who they are. (What a shame to be known in no other way.) When they enter, that’s usually my cue to exit.
Unfortunately, @janbb, I think it would be really hard to set a policy that covers these situations. I don’t think the OP can do much more than redirect the question—although when I try that, usually the off-topic noise just skips over me and goes on. It may be that there’s just no way to legislate cluelessness.
Anyway, the Godwin’s Law equivalent on Fluther is that sooner or later we’re going to end up talking about food. That’s not so bad, right?