Why are there no other indexes in the database structure than the primary keys? Optimizing the structure would greatly improve performance, which would make UseBB more appealing to large sites. For an example, one of the forums (currently phpBB) I host has ~4k users and 100000 posts. With that many posts, indexes would definitely help. Alot.

MySQL Manual wrote
Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

I strongly recommend reading Sitepoint's 'Optimizing your MySQL Application'.

Other than that, UseBB is very, very good :)