For any organization or group that wants to restrict doc access to members of the group, it would be great to be able to create user groups and then give permission at the doc or folder level to the group rather than needing to add each individual. For a group of more than 10 or 15 users, it becomes a real challenge to manage access to docs without such a feature.
Note that folder permissions may have been intended to help with this kind of use case, but 1) folder permissions give all members edit access, which is often not desired for a large group, and 2) in a large organization, it is often necessary to be able to mix and match permission groups at doc level; having some permissions at the folder level and some at the doc level is confusing, at best, in this scenario.
Such group permissions were asked about at Is there are ways to create users groups and attach permissions to them?. I thought it would be good to throw this into the suggestion box.