So, it turns out that when you have selectively added Open Sans typefaces as system fonts, this is what all these sites start to look like: http://i.imgur.com/kgpY6WO.png

I had added two densities so I could muck around with stuff in Pixelmator and was wondering why everyone had chosen the horrible slender version for body text on various sites, but apparently it just means whatever the system collection provides gets priority (which makes sense).