Packing for a high-altitude trek is about layering, not bulk. Focus on a good down jacket, moisture-wicking base layers, and a broken-in pair of trekking boots.
Don't forget a water purification method, a personal first-aid kit, and a headlamp with spare batteries — teahouse electricity is not always reliable above Namche Bazaar.
Most trekkers overpack cotton clothing, which stays wet and cold. Stick to wool or synthetic fabrics throughout.