Until recently, there has been a 250 reply bump limit on threads (the exact number differs on a few boards). After a thread gets 250 replies, it would no longer be bumped to the top of the first page every time it got a reply. It wouldn't ever be bumped again. This was done mostly so that large busy serial threads wouldn't eternally dominate the top of the front page.
The result was that serial threads would spend most of their time on later pages of the index, and the main way to get to them was by following a link in the stickied directory thread. Many less than active threads are in the directory too, so it meant that users aren't able to easily discover active serial threads through the directory unless they already knew the name of a good one to look for, and the serials have much fewer new people coming into them. On the rest of the site, threads are naturally shown to users ordered by activity with brief previews of posts, and it's suboptimal that serial threads are absent from this proven system. It also means that people who frequent the serial threads are trained to immediately click an unmoving link from the very top of the board index, instead of ever exploring the page (and possibly participating in more threads too). On top of all of this, site moderators have to constantly maintain the directory threads, and impose their judgments as to what threads they personally feel warrant listing. A large directory thread takes up a lot of space, probably confusing new users, and many users probably just hide the post and cut themselves off from a lot of site activity just to save the space it takes up. It's weird that we special-case and cripple the discoverability of our most popular threads in this way.
We tried to avoid moving all serials to their own board, but I'm not sure what we ended up with was any better. The good news it that I have plans to fix it, without restructuring the boards or imposing more rules on people. The original issue was that fast-paced serial threads are constantly bumped, eternally hogging the top of the front page. Well, the simple solution is to just make replies bump the thread less often, instead of never.
Now, threads that have more than 250 replies are only bumped by a reply if the thread hasn't been bumped for a random amount of time between 9 and 14 hours. (The random part is to avoid the situation where multiple fast-paced threads that are bumped at the same time continue to bump togeth