As far as heat goes, my Space Gray has not overheated even in direct sunlight in a small private plane, a n environment conducive to overheating due to the more intense sunlight at altitude and thinner air. I think part of the reason is that with the watch in contact with your skin, your body helps regulate the temperature of the device by taking excess heat away. Another factor is the OLED display is far more power efficient than LCD as on an iPhone, so it isn't having to generate massive amounts of backlighting to get through the LCD and filters in an attempt to outmatch the sun.
It should be noted that Apple recommends keeping the watch out of direct sunlight if not being worn... In other words, your body does help cool it.
