The focus in JB was making everything smoother, and they succeeded. Pretty much everything you do will be snappier, and everything just has a more solid feel to it.
Also Google Now if that's your thing.
This shite is fast. Very fast. I flashed AOKP JB ROM. Like Kork said, make a backup as soon as you root(right before you flash a ROM). Make sure you have the right software for the model of your phone before using Odin. After you root the phone you should be able to flash almost anything in recovery.
Still getting used to it. The customization is endless.
Also, Google Now will be groundbreaking when fully launched and implemented. Directions I followed.
But read all you can and watch some videos so you don't frick up. It's easy to get confused so MAKE SURE you create a backup right after you confirm root access, before flashing anything.
This post was edited on 10/17 at 10:15 pm