Support for 1080P video, upgraded user interface, iTunes Match, improved iCloud support with movies you've purchased, Photo Stream so that pictures you take on an iPhone/iPad are automagically on your Apple TV.
If you already have a previous iPad, whether version 1 or 2, and you don't want to upgrade then just don't ever look at the new iPad's retina display. If it's anything like the difference between an iPhone 3GS and an iPhone 4 - once you see the new display your old iPad's screen will look pixellated and fuzzy, particularly when viewing text.
Reality Distortion Field Version: If you don't already have an iPad and have been holding out, this is the version to finally take the leap and jump in with.
Normal Reality Version: You'll be fine without an iPad, new or otherwise. Pick up a used iPad 1 or 2 to play with iOS if you're curious.
The iPad really is a world-changing device whether you buy one or not. Especially when you consider it didn't even exist 2 years ago.