It will probably record live tv and that content will be streamable/downloadable/queued to other devices.
It will probably stream live gameplay footage.
It will most likely have the capabilities to stream network attached content to compatible devices (phones, tablets, other tvs, computers, etc)
If they will have an application marketplace, a PLEX client will be there fairly quickly I would imagine. They had a W8 client available very shortly after release. It will be much better than the Roku and GoogleTV PLEX clients due to the power this thing will have.
Perhaps remote play for some games?
Skype.
Im sure they have thought of lots more capabilities for this thing.
Recording Live TV is useless when most people already own DVR's or just use TV catchup services.
Streaming network attached content is nothing new, PS3 already does this, and I am sure PS4 will do this as well.
Application marektplace is nothing new, things like Roku, AppleTV and Smart TV have been doing this for ages, I am sure PS4 will do this as well. All themain and popular applications that matter like Netflix will be or are on most devices
The only unique thing MS has is Windows 8 client, and sharing the apps between Windows 8 devices. However I don't think this is a big deal, after all Windows 8 hasn't really took the world by storm on their PC OS, Tablet or Mobiles.
EDIT:
People saying MS will have a big hit due to media box features with mainstream market have no idea. Media features are a bonus, what the Durango will sell people on is what games consoles always sell people on. GAMES.