As you can see from the variety of opinions expressed, it really depends upon the person. As for me, I will never, repeat never, take a trip to WDW again between the end of May and the middle of October.
The heat is just unbearable to me and as much as I love WDW, it pretty much ruins my trip. I mean, the heat and humidity are just unbelievable! It's hard to describe really, but here's a few hints based on my personal experience...
You start sweating immediately after walking outside and your clothes are soaked through shortly thereafter. You reach for your bottle of water, and it's no longer cold, in fact it's almost boiling hot, but you chug it down anyway because you are in survival mode and must have liquid of some sort.
You spray yourself with your water-spray fan-thing, but again, you've just squirted hot water all over your face. All around you, kids are crying and miserable, little babies are rolling around in their strollers with sunburnt faces, and moms and dads are at each other's throats and screaming at the poor little kids that just want to go to the pool.
People are dragging around, soaked in sweat, reminiscent of the Bataan death march. Or maybe "Bridge on the River Kwai" when the soldiers had to stand at attention in the hot, blistering sun until they fainted.
You want to get a lemonade, an ice cream, anything cold - but everyone else wants the same thing, and you have to wait for an eternity in the sun, flirting with sunstroke, sipping your boiling hot water, while you wait your turn for the life-sustaining cold substance. You don't care anymore what you see or do, as long as you are in the air conditioning. You wonder if you can just hide in the big ride vehicle at Ellen's Universe of Energy for the rest of the day because it's so deliciously dark and cool in there.
Finally, you give up and go back to the hotel, but the pool is also boiling hot, so you take a shower and a nap, and go back out to the park that night, thinking that it will be much better and you'll get to do more. But alas, humid is the night and sweaty/sticky is again the result. So you take another shower when you get back to the hotel and fall exhausted into your bed, thanking the higher power of your choice for the wonderful person that invented air conditioning.
Anyway, that's my summer-at-WDW experience. The word "hot" just doesn't do it justice. It's Dante's fifth circle down there in the summer.
Your mileage may vary, of course.