We went last July and it was hot and humid. Rained really, really, really, really hard one afternoon when we were at MGM. Remember being stuck in the exit "tunnel" with many other people who were afraid to go out in the rain. I wasn't afraid...I was just waiting to baby swap and had no where else to go.
That said (and to echo others), there are longer hours, fireworks, etc.
We bought ponchos for everyone. DS's poncho was yellow and we wanted to make him stand out, so my mother drew a person on the back. That way in the swarming sea of yellow children we could identify him from the back. Think we got ours at Target. If you want to invest a bit more in ponchos, I got a great one about 6 - 7 years ago from Eddie Bauer, Land's End, LL Bean (one of those places). It is royal blue and they also have a raspberry one. I look like a giant blueberry and no one every loses me in a crowd. Then it folds up in its own little pouch. Very sturdy since it is more of a material as opposed to plastic. Been to Europe and numerous Disney trips...
Have a great time and remember to take plenty of rests.