Jackets and pants must allow perspiration vapor to vent while blocking wind and rain. The material must reduce heat loss and assist the rest of your layers in keeping you dry and comfortable. Most people wore rubber or plastic suits for rain protection, but these materials trapped sweat, made the insulation layer wet, and kept the wearer chilled. Because they weren't breathable, your options were to get wet from the elements or get wet from your own sweat. Able to withstand the most severe rain and snow while allowing perspiration vapor to escape, GORE-TEX fabric has proven to be durable and windproof for a wide range of physical activities.
Today there are several effective performance fabrics available.
In the process of making your choice, consider your activity and the level of protection you expect. Make sure the shell is large enough to fit easily over the other layers and that it is easy to care for so that you can maintain its performance over a long period of time. Use it in conjunction with the other elements of layering and, with luck, you'll never endure another uncomfortable day on the mountain. Heat from Head to Foot Layering works, but you'll only be comfortable and safe from the elements if you make sure your extremities are protected from the damp and cold as well. Body heat escapes through your head more readily than hands or feet.