Another benefit of going at less crowded times is the hotel rooms are much less expensive. A room at a value resort can go for up to $160 per night in high seasons and that same room will cost $82 in value season (and these are "rack rates"-special deals can bring this price down even further-more on this later). Plus, off-site hotels will really have some good deals when demand is down. Value Season can be a good time for you to be able to afford a luxury trip at a Deluxe resort also. Rooms at Disney's Polynesian Resort, for instance, are around $365 per night as opposed to $610 per night for that very same room during Christmas! Add better availablity of discounts to those calculations during value season and you have the makings of an extravagant vacation at the lowest price possible.
The Fall is the absolute least crowded time to go in my experience. I have been during September where the park is literally deserted and those have been very fun trips! However, there is a tradeoff for going at a less crowded time: Disney chooses to refurbish many rides during the slow season. So it is very likely a "favorite" attraction will be down.
This is how deserted a September morning can be:

The weather is also an important consideration. Walt Disney World can be extremely humid and hot-almost unbearably so (and I'm from Texas so I KNOW Hot!) Cooler weather can make a trip so much more enjoyable. Generally, the weather is nice from November until April. It is very hot in the other months and in late summer and fall you can have daily afternoon showers or thunderstorms. These Florida showers are usually short and can provide a welcome relief from the heat but it gets much hotter and more humid after the showers. An important rule is to always have a rain poncho in your park bag so the showers don't slow you down!
Our next trip is planned for May 29-June 4 (my daughter's birthday is June 4th!). The Crowd Calendar has just been upped to pretty much all 8's for that time (from previous 5's and 6's-these are on a scale of 1 to 10). No need to panic! Yes, it will be crowded but given a proper touring plan you can do all that you would like to do and have a great trip!
There is another consideration for when to go: special events Disney offers throughout the year. Here's a brief list and how they can impact crowds:
January-WDW Marathon This has a minor impact on crowds-not a reason not to go at this time.
March - May Epcot's Flower and Garden Festival A wonderful festival during a great time to go (with the exception of Easter week). If you love gardening this is a must-see. Weather is usually perfect.
Easter Week Historically this is an extremely busy time to go, if you can avoid going at this time, I would.
Late May-Early June Star Wars Weekends Are you a Star Wars Fan? Meet the stars of Star Wars and many of the characters at Hollywood Studios. The weekends at the studios will be very crowded during this time.
Early June Gay Days A gathering of gay people-They usually pick a specific park each day and crowds can be extremely heavy at that park on those dates.
Here's a shot of MK Gay Days 2009:

Week of July 4th Can be a very crowded time to visit
Sept & Oct Mickey's Not So Scary Halloween Party A great hard ticket event (usually around $45-65 per person) with many extras during an historically less crowded time. Halloween night usually sells out and can be extremely crowded.
Oct - Nov Food & Wine Festival Weekends can be crowded with locals but this festival generally spans a less crowded time at WDW.
Thanksgiving week A very crowded time to go!
Christmas and New Years A very crowded time to visit with many wonderful decorations
So, the timing of your trip is important but not so important that you stress and second guess your timing after you book and your trip plans are in motion. A fun day can be any day in WDW-crowded or deserted. Some of my fondest WDW moments are during crowded times like Christmas! Whatever you decide, make a plan and stick to it! All of your future trip plans will hinge on exact trip dates!
Happy planning!
