When there's unread posts in a thread, the bars on the left light up either red or blue. Can anyone tell what makes some threads orange and others blue?
I also see it in the grey bars that show "no unread posts", some of them look like they're tinged a bit orange.

I noticed that too, but have no idea either.

Blue threads have posts that you haven't read in them. Orange posts are "hot" and have had more traffic in a given amount of time than is considered average.