We took our TV camera to Anderson Carpet & Home in Burlington and got some insights into what makes good and better carpets.
The real measure of durability is the density of the carpet. With a dense carpet, you shouldn't be able to sink your fingers through the fibers right to the backing. Also, fold the carpet back and see if you can see the backing. You can always see a little, but if you can see a lot of the backing, you know the carpet is low density.
Some carpets have continuous threads. With these carpets, you should avoid using vacuum cleaners with carpet beaters, or you can catch a thread and create a "run" in the carpet. Make sure you are aware of what kind of carpet you purchase, so you know if there are any limitations on how to keep it clean.