Most of my trips to WDW have been in August.
You can expect it to be hot. Really hot. And humid. Really humid. You can also expect that there will be a thunderstorm at some point during the day, usually between 3 PM and 6 PM.
Depending on what part of August, you can expect it to be pretty crowded, although it seems from my experience that the later in the month, the less crowded.
In spite of this, I love to go in August. While crowded, it was less crowded than when I went in July. Also, everything is open later.
I like to stay on property to be able to take advantage of Extra Magic Hours and to be able to get back to my room quickly for resting up in the heat of the afternoon.
I am from Washington DC, so I am used to hot and humid, so the August weather doesn't bother me. Neither do the PM thunderstoms. It is either a useful time to go back to rest, or it thins out the park I am in.