An unread topic is a topic that has received a reply since you last read it. Once you read it, it's marked as read until a new reply gets posted. Along with cleaning up the topic pagination on forum pages, I'll also be implementing a link to the first unread post.

Right now, each user's topic view time is saved as a timestamp--one for every topic that you view. In order to mark an entire forum as read, I'd have to save a timestamp for your user account for each topic in the forum. That would mean hundreds of superfluous records.

I guess I'm not seeing the purpose of the "Mark Forum as Read" function. I've seen it on other forums, but I never really got the point. Can you explain why you want it?