The most important differences in belts are length, width, and ply.
Length - Treadmill users who will just be walking and those who have short legs don't need belts as long as the belts needed runners and taller people. When a belt is too short, the user's stride will feel cramped and this may cause the user to unintentionally hit their feet on the front of the treadmill. Slow joggers will do best with a belt at least 50” long. Faster runners and people over 6' 2" will do better with even longer belts. The longest belts available are about 63".
Width - Belts that are too narrow can cause the users' feet to hit the side of the treadmill. This can cause the user to have to concentrate too much on foot placement. Runners and heavy people need wider belts than walkers and thin people. Runners will want a belt at least 18" wide. Treadmills with wider belts become difficult to fit through doorways. The heaviest people should look for the widest belt they can afford.
Ply - Most home treadmills are made with a single ply (layer) belt. That is, it is one solid piece of rubber. Higher quality treadmills have belts made with two plys. 2 ply belts are quieter, offer a bit more cushioning, and have a bottom ply that is made of tougher material so the belt lasts longer. The highest quality treadmills, found in gyms, have several plys.
Other things to consider in belts are:
Orthopedic Belts – Some treadmill brands may also offer models with orthopedic tread belts. Such belts are designed to provide additional cushioning for people whose feet or joints require it. A similar effect can usually be achieved by adjusting the treadmill’s cushioning to a softer setting. However, If changing the cushioning setting is insufficient to achieve the desired result, an orthopedic belt can be helpful. Orthopedic belt should only be used on machines with larger rollers, as orthopedic belts can produce substantially more heat than thinner belts and the larger rollers can keep this heat level down. See Treadmill Rollers for more information on this.
Lubrication - Most treadmills come with belts pre-lubricated at the factory, so that additional lubrication is unnecessary if the treadmill has been kept reasonably free of dust. However, after years of use, additional lubrication between the belt and rollers may help, as described in the user's manual for a particular model.