Good question!!! Here are a few for me, that I look for. Then gave my reasons why.
Overshot jaw (parrot mouth) or undershot jaw (sow mouth)- Because both defects can affect the chewing of the horse and the horse's ability to clip grass.
Mutton withers- Because if mutton withered, the horse has less range of motion when extending the head and back muscles, so is less able to elevate its back with its head and neck extended, which affects ability for collection.
Narrow chest- Because a horse's ability to carry weight is dependent on the size of its chest, so a horse that doesn't do well with draft work but may be fine in harness or with light rider.
Knock-Kneed- Because it may cause soundness problems in the carpals or supporting ligaments. Horse also tends to toe-out, causing those related problems.
Back at the Knee- Because it places excess stress on the knee joint as it overextends at high speeds when loaded with weight. Backward angle causes compression fractures to the front surfaces of the carpals, and may cause ligament injury within knee. Worsens with muscle fatigue as the supporting muscles & ligaments loose their stabilizing function
Toe-Out- Because causes winging motion that may lead to injury around fetlock or splint.
Pigeon-Toed- Because pigeon-toes cause excess strain on the outside of the lower structures of the limb as the horse hits hard on the outside hoof wall. This often leads to high or low ringbone. The horse is also predisposed to sidebone and sole bruising.
Downhill Balance- Because increases concussion on the front legs, so the horse is at greater risk of front-end lameness. Greater jar on the rider.
Straight Shoulder- Because the horse has shorter muscular attachments that have less ability to contract and lengthen. This shortens the stride length, which requires the horse to take more steps to cover ground, and thus causes a greater risk of injury to structures of front legs and hastened muscular fatigue. An upright shoulder may cause a rough, inelastic ride due to the high knee action. It increases concussion on front limbs, possibly promoting the development of DJD or navicular disease in hard-working horses. The stress of impact tends to stiffen the muscles of the shoulder, making the horse less supple with a reduced range of motion needed for long stride reach.