What makes visiting Disney in the fall better than the summer? Shorter lines (school's in session), cooler weather, and (for FL residents) discount admission!
What more can I say about Disney that you don't already know? If you don't have any kids, or have small kids that aren't in school, this is a great time to visit Disney. You'll still see plenty of families, just not as many as in the summer. And since the weather is cooler people won't be as cranky.
If you have the time and the resources, you have to check out Cirque du Soleil La Nouba. If you've never seen a Cirque du Soleil performance you don't know what you're missing! More information can be found at http://disneyworld.disney.go.com/wdw/moreMagic/moreMagicLanding?id=DTDCirqueDuSoleiLandingPage or once you get to Disney.