I didnt realize it until 10 days ago (though I meant to ask long ago how points were awarded). It skirted my consciousness and I think I saw it as in the IE newsreader and in XML and Swing trees, a way to collapse and expand the conversation tree.

If YC had picked the "standard" icon - thumbs up/down, they would have had to compromise their minimalist Gmail look. But would this have made for a better UI?

(I wonder if there is an "Aha" lesson trying to get out...)