You can do a lot to it if you jailbreak (which is very easy using Seas0npass).
Install XBMC, a media player for videos and music stored on a local computer drive or network drive. Also NitoTV allows packages to be installed, similar to Cydia. So you can get Hulu, Amazon, Icefilms and other streams once you get familiar with it. There are also numerous plugins. These are the two biggies and have the most support.
At this point the mini USB port on the ATV2 has not been enabled. More specifically, the ATV2 is a host device, so it's meant to be plugged into a computer for iTunes to restore it. Likely it will never be enabled.
I just read a thread somewhere that someone was able to enable the Bluetooth stack on the ATV2 and successfully got an Apple Bluetooth keyboard hooked up. This would obviously open a lot of doors and possibly allow headsets, controllers and other BT devices to be connected.