Microwaves tend to heat food unevenly. Since they work by jostling the water molecules, places where the water is more concentrated tend to heat faster than other places. In a bowl of soup, you'll get pockets of boiling hot broth alongside barely warm veggies. You might also see steam rising from the surface and think your food is done, only to get back to your desk and realize it's still stone cold in the middle.
The rotating tray in most microwaves helps, but doesn't do the entire job. Stirring helps to distribute the heat and make sure the food is re-heating evenly. Going in short intervals gives you more control and lets you to check the food's progress so it doesn't overcook. Both things also prevent those messy food explosions when steam bursts from one part of the food and splatters sauce all over the inside of the microwave!
What other tips do you have for re-heating foods in the microwave?