I've not done any real testing and they're barely worth mentioning, but just a couple of things I've noticed:

Reading a poll thread doesn't seem to mark it as read, only the 'mark all as read' button will unbold it.

The 'mark all as read' button seems to apply to a computer, rather than an account? IE, If I mark all as read on one computer then login on another, the threads I'd marked as read are bolded again.