Here's the full list of features via 9to5Mac :

• The device will tie in MobileMe with an Apple built-in Dynamic DNS system. For instance, if your MobileMe account was "9to5mac", your domain name would be "9to5mac.me.com". This is how you'd reach your filesystem throughout the Internet. Your media would also reside here.
• Built in AFP filesharing for Time Machine backups and centralized file stores.
• iTunes shared Library Database for not just music, but also video. Access over the Internet to all of your media.
• Photo database and tie in with with MobileMe.
• Sharing media with iPod Touch and iPhones both over the Internet and while at home. AppleTV as well.
• The box will be based on the Time Machine platform (ARM), not a more expensive Intel/Leopard Server configuration.
• Lots of power saving features like turning off hard drives when not in use.
• Multiple hard drive configurations would be made available with RAID 5 redundancy.