well there are a number of improvements in JB. The 3 big ones are
1) Notification actions - From the notification bar you can perform certain actions if the app allows. Such as end a call, replly to a text, play/pause skip music.
2) project butter- The interface has been given a boost in speed so the whole os feels buttery smooth
3) Google now (which can be hacked into ICS) is a new search experience that has 2 components -
Cards, which is googles attempt to help you, be it traffic stats, driving directions, weather. Lets say Google knows you have an appointment across town, and with gps and traffic knows it is 45 min away, it will prompt you 45 before appt to go.
the other facet of google now is enhanced voice search and voice actions.
There is more, but those are some of the highlights.
And yes cm10 looks to be the only real option for JB on Droid 4