Long sleeve, two pockets w/ button flaps (so goodies will stay in them), either all synthetic or blend (varies with the brand). Team up with a good wicking T-shirt (capilene is great for that purpose).

That pretty well describes a basic, versatile shirt. Layer up for cold, blustery and stormy conditions; roll up the sleeves for warm days. In really hot weather, you might go with an all cotton, loose fitting shirt that will provide sun protection.