What can you do with an Jailbroken ATV2 compared to a unjailbroken ATV2 ?

Check the awkward tv wiki

Here's a summary of 3rdparty software that can run on a jailbroken TV2.

http://wiki.awkwardtv.org/wiki/AppleTV_2#3rd_Party_Apple_TV_iOS_Software

Running a Plex client to play streamed video from a Mac running the Plex server is probably the most interesting thing you can do for the moment.