Humans are infamous for their groupthink in general. The Slashdot variety is actually pretty mild, if you have any baseline for comparison.
The telling thing is that people complain from all sides of various issues. For example, if you ask a libertarian, they'll tell you that Slashdot groupthink is liberal leaning socialist. If you ask a liberal, it's libertarian. But truth is, you see +5-modded comments from practically any perspective. About the only way to be consistently downmodded here without being a troll (or sufficiently troll-like in behavior, even if not deliberate, to make no difference) is to be a hardline creationist.