Q:

What is Ubuntu One? 

are there any paid subscriptions available for Ubuntu One? 
What additional features does it have? 

A:

Ubuntu One is an online file hosting service.  It offers 2GB of space free with an option to upgrade to 50GB of storage for $10 a month. 
With the Free Version you can:

Sync up to 2 GB of files, contacts,
notes, bookmarks, purchased music,
and Gwibber broadcast messages
Automatically sync your digital life
to your personal cloud and with all
of your computers Mobile Contacts
Sync
Mark any directory in your home
folder for sync
Share folders with trusted contacts
or publish files to the Internet with
convenient short URLs
Sync purchased songs from the Ubuntu
One Music Store
Integrated with your Ubuntu computer
Convenient web browser access to your
personal cloud

With the Paid Version you get:

50 GB total of storage to sync more
of your digital life
Keep your address book updated when
you're on the move with  Mobile
Contacts Sync 
Mobile sync supports
thousands of phones including
smartphones like iPhone and Android
Synchronize your contacts with more
applications (like Thunderbird) and
operating systems (Windows and OS X)

