I think the group-feature, and the possibility to create public groups are great. There is one thing though that I would think would make it easier to adminster:
I can assign users to a group, but I also would like to be able to assign groups to a user! Something like this:
Select a user
Select witch groups this user would be assigned to.
The scenario I have come upon where this would be very time-saving is when I have several groups (say more than five) and a new users have registred and should be assigned to (some of) these groups.