Heat: It truly depends on where you're from when it comes to the heat. We're from Austin, TX and 100+ weather is normal through September so we're pretty experienced in battling the heat. Now if you're from the NE I can see how it could get to you. I would stay onsite for the Air conditioned busses and a quick ride to your hotel to swim. Try your absolute best to not be park touring between 12 and 3:30 or so. The sun will be at its highest and hottest with the least amount of shade throughout the day. Swimming is a great way to cool off and stretch out your leg muscles before you head back to the park. Make sure you drink more water than you ever thought you'd need to. By the time you get symptoms of dehydration (headache, grumpiness, thirst) its already too late. It always easier to prevent something, than to deal with it when youre hot, tired, and wanting to do other things than sit on a park bench for 45 minutes recuperating.
Crowds: If you go to the parks when they OPEN and have a solid morning touring plan, you'll get through a good portion of the park before noon. (DBF and I always got through more than half). Maximize FP use and get them whenever you can. Because theres more people, the return times will become later in the day faster than during slow times. Its not a huge deal, you don't have to run to the FP machines or anything, just be aware. The crowds are the highest a little after lunch, which conveniently is when you should be at your resort swimming, missing the heat and the crowds.
Later Hours:Later closing times are such a blessing. Besides the obvious more time in the parks, there are usually multiple showings of Spectromagic and Fantasmic meaning less people in each showing, especially the second showing. Also, its cooler at night, a definite plus.
ADRs: Because its a busy time of year, make your ADRs asap. Don't wait until you arrive.