Incline: Most treadmills have an incline feature. Adding incline will give you more of a cardio workout. Find out how to change the incline.
Speed Adjustment: Know how to set the speed and to increase it or decrease it during your workout. You will usually want to start at an easy pace to warm up for 3-5 minutes, then increase to your desired workout pace. Finish with a cool down of 3-5 minutes at an easy pace.
Programmed Workouts: What sort of programmed workouts does the treadmill have? Does it allow you to program your favorite workout? Does it have heart rate controlled workouts that communicate with a heart rate monitor? Varying your treadmill workout is a great way to progress in fitness. Play with the programs and find ones you can use to spice up your workouts.
Heart Rate Monitor or Pulse Monitor: Many treadmills have a pulse monitor, either in a grip or a clip. This can give you feedback on your heart rate, although you can also see some weird results if you don't attach it right. A chest strap heart rate monitor is more accurate, and many treadmills are set up to communicate with them.
Personal Workout History: Does your treadmill allow you to save your workout history? Don't be anonymous!
Calories Burned: Does the treadmill want to know how much you weigh? That allows it to calculate calories burned, as it is very much dependent on your weight. Less weight, fewer calories burned, so don't shave pounds - tell the truth.