The time of year that you choose to visit Walt Disney World is a very personal decision, based on your preference (or tolerance! ha!) for weather, crowds and events at WDW.
Generally speaking, crowds are thickest during U.S. holidays and when school is out -- Summer, Spring break, Easter, Christmas, and so on. If you want to avoid crowds in Disney World, then avoid these times of year.
The slowest times of year are generally January, February and September. January and February is a great time to visit if you don't mind some mild weather, while visiting in September is a great option for families who can pull their kids out of school and still enjoy the Summer weather -- perhaps some water park action, too! Another suggestion -- the first couple weeks of December, before holiday crowds peak.
Disney World has a lot of special events during the Spring and Fall, which are my personal favorite times of year to visit -- it's not sweltering, but it's still warm enough to enjoy a swim or tour the parks in shorts and a t-shirt.
A website I highly recommend for gauging crowd levels is touringplans.com; its crowd calendar is an invaluable resource for determining how busy the World might be when you're there as well as what parks to avoid on certain days.
When is your favorite time of year to visit Disney World? Leave a comment below.