We just returned almost 2 weeks ago (already!)I found it was wayyyy more hot than last year. I have to admit, I think we are done visiting WDW in summer. We loved our long vacation and I will miss not being able to go for more than a week but we were seriously getting on each others nerves at times!!!
I think if you go expecting the worse, you may be plaesantly surprised. i thin that I had no choice but to go in the summer, I'd still go. The atmosphere cannot be beat, the parks are open longer too.
Just enjoy the resort pools in the PM, and go to the parks early morning and evenings. Drink lots of water and have a nice AC lunch. That should help.