Seriously ... Why?
If you look at the official website for Mountain Lion, they list the top new features -- nothing particularly sexy.
It's still a new beta, and has some stability issues, and even if you were a Developer, it wouldn't be a good idea to run it on your only system, or on all of your systems. Most developers use an entirely separate system, or at least a spare hard drive to run the beta OS, and will only run it part-time.
I can't pin down even a single "must have" feature for ML. It's evolutionary, but certainly not revolutionary.