Two of the most important considerations when planning a WDW trip are when to go and what to tote along.
We at WDW for Grownups love to head to the World during the fall and winter, when the crowds are low and the weather is still lovely (if a bit rainy) and the best WDW events are occuring (e.g. the Epcot Food and Wine Festival and Disney's various Christmas and seasonal offerings).
For those of you vacationing without kids, we suggest scheduling your trip for times of the year when school is normally in session in the USA as crowded times at WDW are usually the result of school-mandated vacation periods.
Here are the best links to help you make those decisions.
Disney World Basics' look at how to decide what time of year is best for you to visit.