Not to many people, but can be chilly and wet. Best time is the 1st or 2nd weekend after Labor Day. Summer hours, few people, well staffed, easy to get around in Orlando. Just don't tell anyone else.
Yes Disney World is crowded and busy on Martin Luther King Jr. Weekend. Many schools have Martin Luther King Jr. Day off which means that the kids are out of school for an extended weekend and any time this happens Disney World becomes extremly busy.
After the holidays in December and the New Year, Disney World slows down quite a bit until March or April. If this economic downturn keeps up, the parks should not be very busy at all.
