Most of my trips to WDW have been during July. It is very hot and humid. But it never bothered me (nor anyone in my family). Umm.. if you do go, keep in mind drink water. We'd fill up our water bottles after every ride (sometimes we used the water to "shower" over ourselves to cool off when there were no watery attractions around). If you're staying onsite, take a break and go back to the hotel for a dip in the pool.
Also, the only other thing I can say is.. if you have gone low travel times, where the ride waits are short: you will be miserable in July. Now I haven't been to WDW in the summer since they got the fastpass, but I know I went during spring break, and most of the ride waits were under 45 min (except Peter Pan's Flight).