I guess my family is the exception -- we recognize it's hot and humid, but if you stay hydrated, wear sunscreen and proper clothing, it's no big deal. Disney is always blasting cold air out of the buildings, and personally, I think one of the best feelings in the world is walking out of the heat into the cool fun of Pirates of the Caribbean or the Haunted Mansion.
Someone mentioned thunderstorms -- yes, there is often thunder and lightning involved, but typically these storms roll through in less than an hour on summer afternoons, and cools things off considerably for the evening. If you absolutely don't like heat and humidity, then I guess that time of year is not for you.
I suppose we locals should consider ourselves lucky that there are so many who can't tolerate the summer weather here -- late summer/early fall is the best time to visit the parks for many of us. We like the heat, and the parks are nearly empty. I'd much rather deal with 95 degree heat at 100% humidity than elbowing my way through 40,000 other people in MK.