Quite a bit - but mainly that there is no virtue in entering into an argument in the hope of changing someone's mind. Below the line (and even sometimes above it) one can venture an opinion that, however impeccably researched and lucidly written, will attract a negative knee-jerk comment in seconds.
It's a bit like a popular beach on one of those rare, hot summer days; there are respectable families, sunbathers, dreamers, exhaustingly active types and just a few boisterous and ill-mannered kids who take delight in kicking over sandcastles. There may even be someone playing music quietly to themselves and and a few friends. All in all, it's a nice beach, though, and there's no shortage of sand...