It's funny because I've found that my posts do better on HN without any headings (though I do use shorter paragraphs for the no-headers).
p.s. almost all of my front-page posts were no-headers!
Those posts would all still be on the front page today if they had included headings!
But really, I admit that this is my most subjective point. I feel like headings are helpful, but a lot of the writers I admire don't use them (Paul Graham, Joel Spolsky, DHH).