I've been in late August many times and it may be hot and humid, but the lines really aren't too bad. It's somewhat less crowded because a lot of Southern school are back in session. It will be more crowded than off season, but really not wall to wall people and you'll be able to get on everything easily if you utilize Fastpass when necessary.
A lot of schools in Florida start up again around August 24th, so the locals at least won't be around as much. But there are schools up north that don't go back until after Labor Day, so that's something to keep in mind.
And it will be hot, humid, chance of rain almost every day.... but if you can take it, go for it!
We went August 24th to the 30th last year and it was easier than we ever could have imagined. There were no lines we couldn't handle, (with the exception of Toy Story Mania. We missed out on it, due to poor planning on arriving early and getting a Fastpass.)
We were staying at Ft. Wilderness, and we didn't have arrangements to see the Hoop-De-Doo Revue. When we checked in, the girl at the desk asked us about it and we were able to make reservations for the very next night, (and we had the best seats, right in the middle of the ground floor!)
As the globe at Spaceship Earth will tell you, everyone there in late August is from the Northeast or the United Kingdom.
Africa and Asia in the Animal Kingdom were both open for an extra two hours over the rest of the park, and my sister and I did Expedition Everest six times in about a half an hour.
(I've never visited Disneyland Resort, Tokyo Disney Resort, Disneyland Resort Paris, nor Hong Kong Disneyland Resort, so don't be fooled when I pretend to know what I'm talking about. [But I'm pretty good with the information, if I do say so myself. And you can hit me all you want with Walt Disney World.])