Consider your activity menu. If you're going to be climbing, remember that you need a coat that won't interfere with your harness. Select one with a high waist, or one that rides low on the hips. Bring your harness along when you are shopping, just to make sure they work together. Second, you'll need articulated sleeves that are cut to accommodate a bent-arm position. If the coat is built correctly, the bottom hem stays down around your hips when your arms are raised, and your wrists stay covered, no matter how you position your arms.
Things to Consider
Hood The weak link of most jackets is hood design. Look for a model that will tuck away when not needed, so you don't always have it flapping in the breeze. Zip-off hoods are an option, especially if you don't plan on using the feature.
A well-designed hood should swivel when you turn your head and allow for adequate breathing space when zipped. Check the adjustment straps on the hood. Will your hair get caught in the toggle cords? Can you see out of the face hole when the hood is up, or do you resemble a shaggy dog who can't see because of the hair that covers his eyes?
Ventilation Anyone who has walked uphill knows how fast you can work up a sweat. The most common ventilation systems are underarm zips that can be opened to allow airflow when you are in danger of overheating. "Pit zips" are a priority feature on waterproof/breathable jackets, and less important on shells designed merely to thwart wind. Some jacket designs come with chest zips and mesh-lined pockets, both of which add ventilation. The advantage of chest zips is that they are convenient to open and allow quick access to the breast pockets of your second layer of clothes. The downside is that they can be mistaken for pockets themselves, which can lead to lost car keys and wallets.
Every day, there seems to be a new miracle fabric available to keep you warmer, dryer, cooler, or just more comfortable. Here are some of the ones you are likely to see while shopping for backcountry clothing.
Sympatex: A windproof, waterproof, breathable fabric made out of nonporous polyester. Also produced as a nonporous membrane of hydrophilic film that can be laminated onto another fabric. Sympatex comes as three-ply laminates, linings, and inserts for outerwear, footwear, gloves, and other accessories.
Fleece: Polyester fabric, sometimes constructed with nylon, lycra, cotton, or ceramic blend, that is generally lightweight and breathable with a high warmth-to-weight ratio. Some fleeces may also have wicking properties.
Polartec Windbloc: Polyester fabric with windproof, breathable membrane designed to protect from weather, wind, water, and abrasion.
Gore-Tex: Nylon or polyester fabric that's laminated to Gore-Tex membrane. Designed to be waterproof, windproof, breathable, and durable.
Gore-Tex Windstopper: Polyester fleece laminated to Gore Windstopper membrane that is windproof and somewhat breathable.
Gore-Tex Activent: Membrane laminated to various fabrics for windproofing breathability. Generally very lightweight as well.