Carpet durability depends on several factors, as a general rule the most important factors are fiber type, amount of twist on the yarn, pile weight relative to stitch rate and gauge which determines density. Judging durability based on any one factor alone can be misleading. Even a superior fiber like nylon will perform poorly if it lacks good twist and appropriate density. Other factors that contribute more to aesthetics than durability are color, softness, luster of the the yarn, and overall texture of the carpet. The appropriate pad and good maintenance will always increase the life of any carpet.

Carpets that perform well in a residential setting can be found in many different styles and construction types. Therefore, a good starting point in carpet selection is simply determining the style you like, then you can look at other factors that contribute to quality and durability.

Fiber Types:

Nylon is an ideal fiber for carpet.  It is strong, has good resiliency, and is available in "soft to the touch" fibers. Topical stain treatments are often applied to nylon to increase stain resistance. Nylon is a good choice for all style types and is available in both staple and continuous filaments.
Triexta no longer called PTT polyester, is reported to have the resilience of nylon, greater stain resistance, is more color fast and is available in very "soft to the touch" fibers. Another ideal fiber for residential carpets. Triexta is always continuous filament.
PET Polyester is generally less expensive than nylon and can be made from reclaimed plastic bottles. PET polyester is considered to be less resilient than nylon but more stain resistant. Is available in both staple or continuous filaments and in "soft to the touch" fibers.
Polypropylene sometimes called Olefin, is often used in "indoor/outdoor" carpets and most often in a looped pile construction.  Polypropylene is more resistant to stains than nylon, generates lower levels of static, is usually a continuous filament fiber, but has less resilience than nylon and other fibers used in manufacturing carpet. 
Wool is a natural fiber and therefore can not be extruded into continuous filaments. The yarns are made by spinning short lengths of wool together to make the yarn, often referred to as spun or staple fibers. For this reason it is natural for wool carpets shed and sometimes pill. Wool is more easily stained than synthetic carpet and holds protein stains such as blood and urine. Wool is also susceptible to mold and mildew, and fades in direct sunlight.

Yarn Twist:
Yarn twist, particularly in cut-pile carpet is important to performance.  Generally speaking, a carpets appearance will deteriorate as un-twisting of the yarns occur. Therefore, yarns with higher twist rates (a tighter twist) will retain their original appearance longer. Heat setting of the yarn also helps in retaining twist and appearance. Today, almost all yarns used in cut-pile carpets are continuous heat set yarns.

Density:
In carpet, density ratings refer to a numeric representation of how tall the yarns are (pile height), how tight the yarns are (stitch rate & gauge), and pile weight per sq yard. This number may be more meaningful to commercial carpet buyers then residential end users because many popular residential styles are "shaggy" in appearance and are created by using taller yarns, tufted further apart and therefore equate to lower density ratings. A higher density number does not always mean heavier carpet, the highest density carpets are often commercial carpets that are short, tight or firm, and weigh less than most residential carpets. In the past higher density ratings indicated a firmer or harder feeling carpet but with the introduction of soft yarns this is no longer always true.

Pile Weight:
The amount of yard used to make the carpet, measured in ounces per square yard. Pile weight certainly is a factor in calculating the density of a carpet, and does have a bearing on durability and cost, but judging by weight alone does not address other major factors for durability such as, yarn twist, fiber type, and texture. Comparing carpet based on weight alone can be misleading. While a heavy carpet may feel thick and rich, if it has poor twist and fiber type, it will not perform well.

Carpet Warranties
The length or duration of the carpet warranty is not a direct indicator of quality, durability or performance. Over the past several years the length of carpet warranties has increased significantly while the construction of carpet has remained basically the same. As warrantees increase in length, so do the limitations, requirements, and conditions for maintaining the often prorated coverage. While the warranty is often an important factor in the purchasing decision, it is best to focus on construction characteristics for judgments in durability.