As we tried to explain in the FAQ, the current beta only does manual sync of the Ubuntu One folder, and not of the other folders. For the next release we will be bringing it up to par with the Ubuntu client, but for now, just that.
It's not going to change, at least in the foreseeable future (and on this I can foresee a couple of years into the future). Allowing users to select arbitrary folders outside of their home for syncing with Ubuntu One, which could potentially sync between multiple different computers, opens a large number of usability problems to cover a use case that, quite frankly, isn't all that common.
One of the problems that I remember off the top of my head is that if you try to sync a mount point of a removable device (and quite a few people try to do this), when you remove the device syncdaemon will delete everything; to make it work properly syncdaemon would have to know about devices, detect their removal, things like that. Quite a large effort, and a lot of potential for usability nightmares.
Another problem is if you try to sync a folder with special permissions, ownership or file types in it: think of /etc/, /tmp/ or /dev/ as some of the worst cases. Or any folder you don't own, really. We could simply disallow syncing folders you don't own, but we know for a fact some people are running syncdaemon as root (despite our warnings).
A workaround for you would be to mount (via /etc/fstab, so you're reasonably sure the partition is mounted every time -- otherwise you risk losing your synced data) the /data folder into your home. You could simply move /data to ~/data or, if you have things that have the /data path hardcoded (quite likely), or if you're already used to /data yourself (also quite likely), symlink or bind mount /data to the mount point in your home. If you don't want to see it in your home at all, just make it ~/.data.
Best Answer
Symlinks won't be followed in Ubuntu One for Windows. The way it will work is the same way as the existing Ubuntu One for Ubuntu: you'll be able to mark any folder as synchronised with Ubuntu One. We're working on that right now. :-)