heat & crowds
Well, ScottieBoy, you've got the heat part correct. Actually, that's an important consideration in the lines question as well, as it's going to be too hot to be waiting outside for long - so fastpass is even more critical than usual. It's also going t to effect where the lines are - SplashMountain sounds so appealing when it's almost 100 degrees, but it does to everyone, so the line might be crazy - that's the kind of attraction I'll ONLY ride with a FP, as I've seen summer lines there in the hour range, or even more.
Actually, even though it's so hot and a time I usually try to stay away, I'll be at WDW the week after you, so I have the same concerns. In the past, I've found summer attendance to be surprisingly high/busy given just how hot & uncomfortable it can be, so expect heavy attendance for the time that you are there - but not "peak" type attendance or waits. Also, keep in mind that the typical summer afternoon thunderstorm can do a lot to clear out a line - or the park, for that matter.
Bottom line - it will be busy, but use the FastPass at every opportunity, take advantage of as much A/C as possible, and have fun!