> * the US lacks regional parties, which is another source of third-partyism, but depending on your views this might be a good thing.

IIRC, for a lot time the two major US parties were effectively federations of different regional parties (e.g. your farm-state Democrat had a very different set of positions that your northeastern urban Democrat, ditto for Republicans). That's collapsed in the last 20 years, and everything's basically national, which is a big part of the problem.