Inactivity/Cookie Based - once a user has been inactive for a certain amount of time (the value of the session timeout option) all threads and forums are considered read.
This is the part that is annoying to me. If I come here and start reading threads and come across one that requires a little time to read, respond to or research, everything is marked read by the time I'm done. I could come in with 200 new posts and by the time I read through 1 new thread with more than a few pages or respond to something that takes a little while to type out, the whole forum is marked read and I have to search by date/time to find the other 150 posts that I wanted to read.