I'd like to be able to join two guilds. That would mean having two guild chat channels and being able to pop two different guild tags. That way you would be able to dip into a large guild talent pool and be able to form smaller roleplay groups with your friends if you wanted too. It would sure spice up PUGs by being able to call your self "Apple Dumpling Gang" or whatever.
Also, no other MMORPG has that and it would be one more thing that make DDO the best!