Potentially, the most used Mavericks option I'll take advantage of will be to use a HDTV as an addition screen. If you buy a MBA, also buying an Apple TV becomes a no brainer.
I've been keen on mountain lion from the beginning for the airplay mirroring.
But today I've been playing around with the airplay mirroring in mavericks using the Extend Desktop option with my HDTV as a second screen. It's pretty cool.