If you walk into a gym or health club, you'll see a number of different floor surfaces: concrete, hardwood, mats and tile are just some of the common types. However, many would-be athletes work out on carpet for the simple reason that it's what's covering the floor at home. Although working out on carpet isn't bad from a risk of serious injury standpoint, it does have some disadvantages you should work to mitigate.
Carpet gives under your weight. It's a soft surface on top of a soft surface, which usually rests on flexible wood. This is great for exercises where you lay down or kneel, but it can be a problem doing exercises that require precise balance such as cardio kickboxing or yoga. When the surface gives under your feet, it changes your footing and interrupts your balance. To mitigate this, you can pause briefly as your foot settles into the carpet before beginning the posture that relies on balance.
Anybody who's wrestled with a brother or cousin on the carpet can tell you about rug burn: the abrasion you get from scooting a body part across the carpet at speed. Any exercise that has a lot of floor contact, such as ab labs done on your back or Pilates poses on the knees, run the risk of rug burn. Putting a yoga mat on the ground, or wearing clothing that covers the contact areas, can prevent most rug-burn problems.
It's easy to tell if hardwood or tile is clean: stains and soil show up easily on the smooth, flat surface. With carpet, it's harder to tell -- meaning that you could be lying down to do situps in a combination of cat urine and old food. Allergens are particularly adept at hiding among carpet fibers, making this a particular issue for athletes with hay fever or pet allergies. The best solution for this problem is to vacuum and shampoo your carpet on a regular basis.
Wear and Tear
Carpet's not cheap, and exercising on the same piece of carpet -- for example, the section in front of your television -- can fade and wear that section prematurely. That prematurely worn section will be obvious surrounded by areas with less wear. Though not a problem from an exercise standpoint, this can make a difference in the atmosphere of the room. You can solve this problem in the short term by exercising barefoot, avoiding the extra wear inflicted by shoes. Another solution is to use an exercise mat placed on top of the carpet.