The profile of a well designed windmill blade resembles a aircraft wind, giving lift on the trailing edge. The blade has a angle of attack to the wind to give its best lift, and this lift drives the blade forward. There also needs to be a slight twist along the length of the blade, the blade tip is traveling much faster than the part of the blade closest to the center of the propeller, so the blade needs to be tilted to give the same angle of attack. 
You also need to consider tip speed, or TSR (Tip Speed Ratio). The tips of the windmill blade travel much faster than the wind speed, so a TSR of 7 means the tips travel at 7 times the wind speed. A typical 2 meter diameter turbine at 500rpm could have a tip speed of over 200kmh, and any airborne dust or unfortunate insects will be very abrasive at this speed. For windmill blades made from a soft material, such as timber, a layer of wear resistant aluminum tape or fiberglass is usually applied to the leading edge.
