But downvotes just as often (or more often, I'd argue) create exactly the tribalism you describe. Restrict users to only a positive input mechanism and you'll drastically reduce bullying and other gang behavior.
I guess I don't see this being a problem "all the time" the way you do. Sure, every so often you start seeing lots of articles about the same contentious debate happening in a pretty tight echo chamber (or sometimes you start seeing nothing but Erlang articles at PG's behest), but save for the occasional dud most of what rises to the top around here seems pretty well worth my time.