I would really appreciate the ability to modify a user's group memberships from the user section of the ACP. It's a more straight-forward approach for me to say "this user should be a member of groups X, Y and Z" than to say "group X included users A, C, D, F, H and K." This is especially true when the groups get long. If modifying the group membership from this direction is difficult for some reason, it would get me more than half way there if I could just see them displayed in this manner.
As an extension of this, I would also like to be able to add groups to groups (e.g., all senior members are members and all moderators are senior members and all administrators are moderators). I frequently find myself adding the same people to multiple groups when this would be a much simpler solution.
Likewise, I would also really like the ability to view the category level permissions of a specific group from that group. Specifically, I would,like to be able to look at the members group and see that they are permitted too view and post in Categories X and Y but only read Category Z (etc.). Again, modification from this angle would be ideal, but just being able to see the information presented in this way would help.