I'm always a little surprised when people mention that they have difficulty reading serif fonts, since I don't even notice the difference. My own website uses Times New Roman, and occasionally I'll consider switching it to something else, but the thought of editing 80+ pages always discourages me.

Anyway, I'm with shina_laris: I think the reason badfic is badly formatted isn't (necessarily) that the writers are new to the internet, but that they're younger. Because of that, they're both 1) more likely to choose, um, unique formatting in an attempt to assert their individuality, and 2) less likely to have vision problems that would prevent them from reading their own writing.