It would be very useful to be able to use group membership as a criteria for an advanced search or dynamic group membership in addition to being able to use contact and opportunity criteria.
For example, we use groups membership to show attendees for events we run. Each group shows a different list of attendees. If we want to create a list of everyone who has attended an event in the past year, there is no way of combining this information without doing multiple lookups and exporting. You should effectively be able to create a dynamic group using membership of other groups as a criteria.
Similarly, you should also be able to use group membership as a criteria in advanced search. That way you could, for example, search for contacts who were from a particular type of organisation that belonged to specific groups.