In my opinion (as an administrator) ISPConfig2's best feature was the easy backup's of web/db/mail/log.
If restoration of those could have been implemented it would have been perfect.
After all, backup/restoration is the most important aspect of any system.
It directly translates to why virtualisation is so powerfull. It gives us easy ways to backup/restore/move/copy data.
At the moment this is still taking its baby steps. Only server level virtualisation (mostly). I hope to see the day when we can move mailboxes and websites from server to server like virtual servers can now be moved from host to host.
Imagine the power of such a system.