Threads Clickable Prefixes
 

I don't know how in-demand or useful this would be to others, but I use it a lot. Basically, prefixes of threads on forumdisplay should be clickable. Upon clicking the prefix, I should be presented with a list of threads that have that prefix.

Here's how to implement it: http://www.vbulletin.org/forum/showthread.php?t=169832
I think it might be a good idea for a stock feature.

And if you really wanted to go the route of usability, you should have a small little warning-like box appear offering you a link to reset your filter and go back to the list of all threads.