"Rightly or wrongly some of the most interesting threads are 'nasty' ones."

Heated discussions about real issues can indeed be "some of the most interesting threads", even when at times some people can get overheated and start tearing into each other instead of exploring the issues and the differences in how people see them. I'd hate it if the house rule was that no disagreements should be aired - "no politics, no religion and no football", as it goes in some pubs with a history that explains why.

Bur every now and again the discussion sours, and its time to get out, and that's what I mean.

It's very subjective, and no doubt people draw the line in different ways - but I think most people do have a line they draw. For me it's when a thread seems to have run its natural course, but it's still going, turned from a discussion (however heated) into a repetitive succession of personal insults being thrown around, with the same things being said over and over in an obsessive way.