Microwaves agitate water molecules to heat food. This is why microwaves make food rubbery or hard, much faster than an oven. You want to avoid aluminum in a microwave. It's very reactive, and if it's in contact with other metals, it will fuse and melt. Any material that comprises electric dipole molecules will not be a good thing to be used in a microwave. It will heat and possibly melt.
Plastic will only melt when in direct contact with whatever material is being cooked.
All foods can be cooked in a microwave. The problem with some foods is that they're dryer than others. Microwaves only penetrate deeply, more quickly, by agitating dipole molecules (molecules with a negative charge and a positive charge on opposite poles, such as water).
Use a plate or paper towels to cover your potatoes. If you're steaming, cover it with something that's going to trap the moisture. Either way, a microwave oven is not ideal for cooking anything you want to keep really moist, unless the water content is very high (soups, sauces). If you're reheating noodles or rice or some such food that absorbs high amounts of water to be edible, add a bit of water before reheating. It will help the cooking process, and prevent overdrying.
Avoid metals, but contrary to popular belief, putting a fork in a microwave will not blow it up, or cause fires, or ruin the microwave. It will simply not cook it, and the small amount of waves coming off the magnetron will just bounce off the material, but no massive explosions will occur. Very, very thin or low density metals could in theory become very hot, and perhaps melt. But the amount of time and frequency of the exposure to the microwaves typically wouldn't happen in the average cooktime for any food item.
There are a lot of misconceptions about microwaves, such as that they cook things from the inside out. These are myths. Read more about Microwave Ovens from the sources listed below.
Wikipedia, Mythbusters, Encyc. Brit, World Almanac, The Science Desk Reference.