The fact that you can belong to more then one guild is a great idea, but the implementation could be a lot more then it is today.
It could really change the way guilds are used, by allowing people to join various guilds that represent various interests. and actively participate in all of them. This could result in a far less fragmented community.
To support this however there are two things necessary. #1 is more important then #2
1. Allow players to monitor and chat on guild channels for more then one guild at once.
2. Have a setting to display all your guild tags over your head, not just the one you are currently representing.