Larger party (group) sizes. Up to a max of say 40 just as an example although if there are people who want larger thats fine. This is so people with larger groups of friends can play together.
This would would allow you to group up and share conversations with more friends.
This would allow you to organise your guild members or large group of friends during a elite meta DE.
The ability to see your guild or large group of friends on the map as a dot.
This would allow you to all enter the same overflow (instance) when travelling around the world.
None that I can see, although if anyone has something to add to this area I will post it here.
I know you dont need groups to fulfill the DE's however it would make it a lot easier to keep track of your guild mates as you can see their dot on the map. Also the ability to stay with your group while you travel through the world.
Mainpost edited for clarity.
EDIT1(after input from Roachsrealm):
Another possibility for this would be to integrate the above into the guild system by adding guild members to your map as a dot and adding the "right click to join" function from group members onto guild members.
However this would take the feature away from large groups who arnt guild mates.
Edited by Ubung, 28 August 2012 - 11:12 PM.