The most noticeable change is the interface. The task bar on the bottom has been moved to the left and features easier access to your various lists and daily plans. From the side bar you can swipe through tasks in a new card stack layout that makes managing all your to-dos a bit easier than the previous menus. You can also now edit multiple tasks at once, which makes it simple to postpone or delay items when things go awry. The interface overhaul alone is a big improvement, and the new gestures make for an easy and quick way to navigate your tasks.
The update is free in the iTunes App Store and brings with it a retina display update for the new iPad as well.