I'm currently subscribed to your services and I love them. Had only one problem with Zextras Mobile that was fixed the next day (great support).
To the point:
Live migration (Server duplication)
I would like to have a possibility for a server duplication, so when one server goes down (malfunction or a planned technical upgrades) - second server takes all the load. I know that it is very hard to implement, but that is the only feature that makes a really big difference between Zimbra OS and Zimbra NE!!!