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Material and Feel
Polymer is basically a specially developed, very hard plastic that can replicate the hardness of metal but is much lighter and in many ways more durable. The bulky, low-impact parts of a firearm, particularly the frame and grip, can be made out of polymer, while the moving parts that are heated by the gun firing (such as the action) can remain made of steel. A polymer frame feels lighter. It isn't light, especially when loaded, but it isn't heavy. If you are used to steel-frame guns, a polymer frame may feel top-heavy and off-balance, especially when unloaded. A few readers on The Firing Line consider the weight and balance of polymer frame guns to be not worth getting used to.
Strength
Many people complain that polymer guns don't feel as strong, probably because the material is so much lighter than metal and isn't cold to the touch. Some even say it feels like a child's toy. Steel, they say, feels more satisfying.
In terms of laboratory testing, polymer is stronger than steel at any given thickness. However, the forces that act on a firearm are different than those in laboratory strength tests. Some people say that polymer frames crack more easily than steel frames if, for example, the casing of a round of ammunition breaks and the exploding gases are sent downward instead of out the barrel. A cracked barrel isn't dangerous (with possible exceptions if you aren't wearing your safety goggles), but with polymer the gun is useless. If this problem occurs in a steel gun, however, the magazine is destroyed and the gun jams, but it will remain intact, and once the jam is cleared, functional.
Function
Some people have found polymer-frame guns misfire more often than steel frames. Some say the plastic is sometimes not machined as well as the metal is in otherwise comparable firearms. This leads to misfires and malfunctions.
Many people are also thrown off by the weight and balance of polymer -- as mentioned before, the gun is top-heavy and the feel may be somehow "off" for someone used to heavy steel frames.
Others, however, praise the contoured grips that polymer frames can provide, saying that the shape and texture makes the gun easier to grip with proper firmness. Part of the function of polymer is to absorb some of the recoil. The lighter weight, however, increases the amount of recoil, so the net result is variable.
If you want to add accessories to your gun, know that polymer does not take well to drilling. Drilling into the frame can cause malfunctioning. You will need a tactical mount.
Longevity and durability
Polymer frames have not been on the market long enough for people to know how well they age compared with steel frames. Steel guns have been around for centuries. It isn't difficult to find a firearm, whether handgun or long arm, that's a century old but holds up fine for regular target shooting. Some families are lucky enough to have heirlooms that are hundreds of years old. Polymer frame guns seem durable, and they have promise, but they simply haven't been tested the way steel guns have.
Polymers do seem durable, though. Torture testing of polymer frames, particularly Glocks, has gone well. These tests involve freezing or packing the gun in mud for months at a time, then field-stripping, brushing off the dirt, and firing. A U.S. Marine commenting on Yahoo Answers observes, "All wear contact points are 'steel-to-steel,' the service life expectancy of a polymer-frame weapon will be the same as an all-steel frame design."
Polymer holds up to regular wear and tear better than metal and certainly better than wood. Metal reveals bangs and dents and scratches that polymer just doesn't. Polymer does not rust. Certain cleaners that are appropriate for steel frames, however, might permanently discolor polymer, according to a thread on Ruger Forum and one on XD Talk. There's a chance the chemicals in polymer might react to the gun cleaners you regularly use for metal guns. Discoloration or residue is harmless, but some chemicals may eat at the plastic. Read the label if you decide to go for a polymer frame.
History
Related to the questions about durability is the question of the importance of history. Polymer frame guns have no history. To a collector, they tell no story. Steel-framed guns have helped win continents, fought in wars across the world, perhaps helped your father or grandfather feed his family, and so forth. Many people criticize polymer for not having the same kind of history or memory attached to it the way steel or wood does. The guns are also criticized for being unattractive: "Very few pistols are as ugly as a Glock [a veyr popular polymer frame gun]," says an Epionions reviewer, discussing Glock pistols.