Motors are the heart of a treadmill. Look for a minimum continuous rating of 1.5 HP. Be careful because some manufacturers rate their treadmills as peak or intermittent horsepower. American made motors are preferred (not offshore) and should be fan cooled. Motors with lower RPM (4200 and lower) will be smoother and last longer.
The suspension of a treadmill should always be between the deck and the frame. Avoid designs that use the feet for suspension. Good suspension cushions impact and reduces lateral motion while providing shock absorption throughout the entire running area.
The belt must be long and wide enough to accommodate all stride lengths. Avoid cotton belts. They do not last as long as 2 ply polyelastomer belts.
Frames can be either aluminum or steel as long as they are welded in a 'closed box' design. Aluminum is lighter and will flex more upon impact while steel is heavier and can help dampen vibration.
Make sure the display console is easy to read and operate. Display should show speed, time, distance, calories burned, elevation and have heart rate as an option. Some come with an optional magazine holder. A safety key is a must on all motorized treadmills. Look for heart rate interactive programs.