Oh yeah, another thing I had in mind was a filter of sorts at the bottom of the treeview area, much like the text input in the regular chat area.
This filter box would work much like the one in the DC++ clients, if you type something in the box, the userlist will only display the items containing that string, making it ALOT easier to locate particular users.
@Spitfire:
Don't be so negative. This suggestion will potentially remove the need for the user list all together. And if you don't want to use it like that... just don't. :P
It won't create any 'unnecessary space', it will just relocate the user list from one place to another, freeing space rather than taking it up.
As it is now I rarely use the userlist, and having it collapsed, yet easy to access, would be a great improvement for me.
And should I want it to show only operators... well, just enter @ into the filter box. Voila.