On a forum as huge and with as much traffic as this one, it will make the database crawl to a halt if it has to know exactly which threads you have read, and whether anyone else has replied to it.
On much smaller forums like those typically running something like phpBB, where this happens, it doesn't slow down the forum any, as these writes and reads to the database as you read a thread to mark them as read aren't happening too often.
There are typically 200-400 people logged in and hundreds more just browsing without being logged in. Having the database written to every time someone reads a thread to say it's been read would probably kill the database servers from all the extra work it will have to do 🙂
Therefore, the best way for a site this big to work is by marking everything since the last time you visited (which is stored in your cookies, so no unneccessary database writes) as unread.
Confused