HEHE[16:22] [TMS]GPow69: want another post?
[16:22] [TMS]GPow69: make a topic that "say something about the guy above you" now has 100 pages
[16:23] [TMS]Maffia171/Swizzy: Where?
[16:23] [TMS]GPow69: milestones
[16:23] [TMS]Maffia171/Swizzy: O
[16:23] [TMS]Maffia171/Swizzy: right
Cap the length of your threads This is one of those things that ought to be a setting - even vBulletin is known for choking on large threads, look up rpg.net's motivational poster history. Large threads mean there is a large result set for further instructions in the query to prune from, and if a thread is too disproportionally large, even browsing the thread will result in using table scans instead of the index - a very bad situation indeed.
30k is what I use for Elliquiy, and Ben uses 25k for Red and White Kop. Keep in mind, though, that we are both running finely tuned dedicated servers with two hard disks, two cores, and 4gb of RAM. You may want to cap thread length closer to 1k posts.
This also has the effect of making individual threads less overwhelming.