I originally put this in the food section, but it was suggested that it might be better off here....
When I reheat food in the microwave (such as a casserole from the previous night) I find that it cools down much more quickly than if it had been reheated conventionally in a saucepan or over water.
The utensil that you use to nuke the food in usually comes out a lot cooler than from a conventional oven, but in reality the food is hottest right through from the microwave so, all things being equal, that food should stay hot longer.
Microwave ovens rarely heat evenly and consistantly due to constructive and destructive interference between the electromagnetic waves bouncing around within the heating chamber. In addition, absorption of microwave energy depends largely on water content which is often not evenly distributed throughout the food being heated. As a result of this uneven heating some parts may become intolerably hot while other parts remain unheated. After a short time, heat from parts that were initially heated sufficiently is drawn away by the cooler parts until the heat is distributed evenly. This redistribution of heat may contribute to a suspicion that the food is cooling more rapidly when in fact the heat was not initially evenly distributed.
Thank you all for your speedy answers! I get the gist of it now and understand why it is always suggested that food is stirred during the course of heating.