Next considerations are speed and incline. Any treadmill with a max speed of 10MPH and 10pct should satisfy most people, but if you need more, treadmill speeds can go to 12MPH and incline to 15pct on higher end models.
How many training programs do you need? Some treadmills offer as few as a handful while others offer as many as 50. IMO, once you get to a dozen, you really have all you need. If you are serious about your cardio training, look for treadmill with heart rate programs. The treadmill will then adjust its speed to keep you in your target zone.
Heart rate monitoring is now standard on most treadmills, with contact grips available even on budget models while wireless chest strap on most models starting at $1,000.
Internet connectivity has become a feature on many treadmills and it is now possible to download training programs, upload and track your training sessions to the web, while some treadmills have built-in screens that lets you browse the web while training.
Extra features vary greatly among treadmills and include music docking station with built-in speakers, cooling fans, bottle holders and reading racks. However, there is little correlation between added features and quality. Often the highest rated treadmills have relatively few extra features with the money spend on quality while lesser models throw lots of extras at you to compensate.
Finally, warranty matters greatly. The length of the warranty is a great indicator of the quality of the treadmill, no manufacturer is going to offer a warranty that they expect will cost them money. Unless you are on a strict budget, look for a treadmill with lifetime warranty on the frame and at least 20 years on the motor. On parts and labor, look for 1 year on both. Some manufacturers are willing to go to 3 years on parts and 2 years on labor. While that may cost you a little more, you also know that the manufacturer believes in its treadmills.