It's all about how you plan your trip
If you blindingly see the world in the summer heat, then you could be severely impacted by the heat. Then again, if you plan ahead and use certain key strategies, your stay will be much more pleasant.
1. As stated earlier, mornings and evenings aren't that bad. Evenings are usually very pleasant. Mornings are fine, so long as you get up early and are at the parks when they open.
2. Drink plenty of water. You can find water fountains everywhere in WDW
3. There are several water rides which will cool you off nicely: Splash Mountain, Kali River Rapids are the main ones
4. Walk through shops and fast food restaurants throughout the day. They provide air conditioning, and are usually entertaining as well.
5. Some rides have more indoor queue than others. For example, the Pirates of the Caribbean queue is almost entirely indoors, and entirely shaded. On the contrary, the queue for Aladdin's MAgic Carpets is entirely in the sun. Most queues stretch outside, but are covered and have fans and maybe even misters.
6. Don't be afraid to hit some of the smaller attractions. They'll provide entertainment, and a welcome break from the sun.
7. In Epcot, many of the attractions are in huge buildings themselves. You can walk into many of them without even going on the attraction itself. (Wonders of Life, Mission Space (interactive exhibits via the ride exit), Test Track (interactive exhibits via the ride exit), Journey into Imagination, The Land, The Living Seas, Innoventions
I could go on and on here. There are plenty of things to do to make your stay much cooler and enjoyable even in the hot Florida sun