When I think WDW in the summer, I think crowds, extreme heat, and lots of crying children and screaming parents. Yes, screaming. I have seen more instances of parent-child melt downs at WDW than I see in a whole year anywhere else. I don't know if it's the heat, dehydration, or what - but it doesn't seem to bring out the best in people.
Supposedly Orlando sits slightly below sea level, so you get a "bowl effect" in the summer where the heat and humidity just hover endlessly - except for the requisite afternoon monsoon (which hits like clock work, typically when you're in the middle of a park with no quick escape route).
I'm spoiled because we live relatively close and can go in the off-season.
If you really want to go please think about this book: http://www.amazon.com/Unofficial-Guide-Disney-World-Guide... . My sister and I swear by it - will give you lots of great ideas and tips on how to stay one step ahead of the crowds and how to hang on to your sanity.
I also recommend the water parks - that's what we tend to do in the summer. Either get there 20 minutes before opening, or go late in the afternoon right after a storm.
Good luck and have a great time if you go. You can still pull off a fun trip with some advance intel. :)