For Ubuntu One to succeed it has to have some kind of advantage over the rest of the dropbox clones.
But in fact it doesn't and lacks behind those other services which are cheaper, have better client side applications, are fully cross platform. They all use the same base principle, storing data in the "cloud", but none of them let users / administrators use there own hardware.
Even for me, as a 99.9% ubuntu user, it doesn't offer me enough to even consider it as a alternative to dropbox or simple webdav / ssh sharing.
The only thing that would make it worthwhile besides it competitors / alternatives is to offer a "host it yourself" option which would open a whole new market. Not everyone wants there data to be stored "somewhere".
To make it a no brainer service which would also help ubuntu gaining market in the office space it should include the following features ordered by importance:
1. It should offer different cloud services, even ones hosted by yourself (As default package within ubuntu server).
(I agree with Fred although the vendor lock-in doesn't seem to be as apparent as he makes it look like)
2. It should support sync over LAN if machines multiple machines are detected using the same share.
3. It should support multiple shares (folders).
4. It should share software repositories cross ubuntu clients and make profiles which would install/remove software from attached machines automatically (ubuntu only).
5. It should offer a centralized user management for ubuntu clients (ubuntu only).
(install ubuntu, define server in the installation procedure and select machine profile)
6. It should allow sharing without data syncing / replication (Media files for example).
7. It should offer a website with login and generated one time hashkeys to be able to reach and share data over the web.
8. It should use existing data sharing options and configurations on a plugin bases (Firefox sync for example).
(could be as simple as syncing a single folder from you home folder)
9. It should have a nicely integrated version control system in the explorer (nautilus) as well as through the website.
10. Possibly a server version for those NAS devices out there (Synology for example).
Just my 2 cents.