772MB of downloading gets you to Android 4.3 Jelly Bean and all the goodness that goes with
We don't currently have a list of the changes (the update itself isn't "official" just yet) and the update just started pushing so there's not a lot of customer feedback for now. What we do know is that 4.3 is bigger than 4.2, therefore the laws of smartphone physics say it must be better.
If you have an S4 Active from AT&T, be sure to check for your update. Discuss in the S4 Active forums!