We were there Aug 30- Sept 6. It was HOT! The crowds were perfect...walked right on most rides, especially after Labor Day (even LD weekend wasn't that bad).
It is hot and humid but I notice the sun is a lot stronger down there, even in September. It can be pretty miserable after a while outside, but a dip in the pool can really rejuvanate you. I always plan on at least 2 showers a day when in Florida.