Is there any plan in future to let users rearrange the order manually (Like you can with browser or taskbar tabs, for example)? Maybe create a list of 'favourites' so if you're in a microcosm with shitloads of forums you don't have to hunt through multiple pages for the one(s) you want.
Forums are order by criteria:
The first 10 or so forums on the front page are stickied so that new visitors get a really good idea of what "on-topic" looks like.
Everything after that isn't sticky.
Within the first 10 (or so), and the rest... it's all sorted by popularity (comment count and item count).
We have a feature fully ready, to allow people to create their own forums. When we open this up the number of forums will grow rapidly.
Examples of things I imagine will become forums:
Given that a lot of forums will be created, they will be auto-sorted by popularity.