It's gotta be in the height of summer July/August time, and other holiday breaks e.g. Easter Holiday and Christmas Holiday, whenever the kids are off school I personally would stay away from WDW and any other theme park for that matter.
I went last September, although the parks wern't very busy (except MK Sept 2nd) and the queues were short, the weather wasn't very good, we were often caught in storms and rain showers, kinda put the dampers on the holiday really (no pun intended). I guess those of you with children have to go in the school holidays unless you are going to take them out of school.
My first visit to Florida was in May 1997, the weather was excellent no rain, but it was hot (high 20s low 30s from what I remember). The parks were not too busy, the longest queue was 45mins for Splash Mountain, the others were usually 10-20mins which I think is great for somewhere as popular as WDW. I am told that April is a good time to go (after Easter). The only thing about the low season is that the opening times are not as long, and the parades/fireworks etc do not show as much.... something to be considered.
My next visit I plan to go late April / early May time.