It would be REALLY helpful to be able to select and delete users based on criteria.
The application that grabs my current attention is getting rid of spam entries. For now I'd be happy to be able to delete all Unconfirmed users. Though, for greater control, one might prefer to delete unconfirmed users more than a couple days old (so as not to lose legitimate signups who just haven't gotten around to confirming yet), or unconfirmed entered within a date range containing specified content (such as html in a textarea field).
I'd guess some of us might prefer not to bother keeping "blacklisted" users in the list, so being able to select/delete them might be valuable as well.
I'd like to see a way to "Tag" users by selection criteria, and then be able to "move", blacklist, or delete tagged users (I mean REALLY delete them, not just remove them from a list)