The ability to be microwaved. The truth is that just about everything in our universe is microwaveable. On the other hand, many things are not microwave safe; that is, they can be safely cooked inside a microwave without causing damage either to itself or the microwave.
For the most part, almost all food is microwave safe (although it can get messy if improperly covered.)
Most modern ceramics, hard plastic, and glass is microwave safe, but they should be clearly marked as such on the item itself or on the original packaging. Check carefully before stuffing your nuke box.
Soft plastic such as saran wrap and chip bags are microwave safe, but will melt and warp with the heat. Thus these materials should not be placed in direct contact with the food being cooked.
Most Tupperware is not microwave safe. This also goes for most paper products, including paper plates. New paper plates have certain coatings that can prevent warping, so be sure to check the packaging.
Finally, metal is not microwave safe. Cans, aluminium foil, silverware, and yes, compact discs will all spark and crackle in the microwave. Large amounts can severely damage the internal workings of the appliance.
In closing, check the original packing or the item itself if you are unsure of its microwave safety. Mmm ... burrito ...