Large threads pose a heavy load on the server as they queue up lots of pages. Nothing wrong with the thread concepts and new ones can be started, just they'll be closed occasionally to manage things better.
Nicko, I was wondering... you mentioned that people can use those old threads to up their post count... are there features of the site that are only accessible to people after they have made x number of posts? Just curuous, really. I was part of a forum a few years ago that had some of its' areas only accesible to people after they reached a post quota. Frankly I disagreed with the entire thing because people were posting just a smiley or LOL in order to reach the magic number. (needless to say I stopped going to that site as most of the posts were just there to reach the magic number)