This would be a handy feature for many users, especially people who are building multiple lists in multiple niches.

The idea is simple. Allow folders to be created which we can add lists to. Then, when someone wants to send a new campaign to all of their lists in a certain niche they don't have to go through all of the lists holding ctr and clicking. They just click on the relative folder and that auto selects the lists contained within in for that campaign. So then in the pane where lists are selected to send the campaign through there would be the familiar tree structure just as you have in windows - clicking a + next to the folder would expand to show the lists inside so specific lists could still be selected but, if the campaign is to that entire niche group, then just the folder could be selected and the mail is sent to all the lists in that group.

This would also help people create list groups of customers - people who have bought something - or lists of people who open mail regularly. Then the most responsive lists could be contained in separate folders so that more targeted mails could be sent.

Just my idea to enable people to better organise their lists, make it easier for them if they have many targeted niches and make targeting more accurate.