Stability of the treadmill is very important. You should look for a sturdy frame made of welded steel. Some more expensive models have X-shape braces under the belt, that will enhance the stability of the unit. It's not something you can easily see, even at the store, so ask your salesman about it. Also, the treadmill should have a shock absorption system, so you will be able to run without injury. An orthopedic belt can also greatly benefit runners with bad knees. The length and the width of the belt have become a standard, but beware of cheap models with short belt. It may results in an injury if you fell.
The quality and the quantity of the programmed workout is very important so you won't get bored. Bodyguard's T460XC have 14 programs so everyone can keep entertained. You can choose between weight loss programs, activity specific programs and intervals.
The warranty of your next treadmill is very important. Look for a treadmill that has a lifetime warranty on the frame and motor, plus a good warranty on parts and labor. It is particularly important to protect your investment. Bodyguard's treadmills have the best warranty of the industry. A good treadmill doesn't ask for a lot of maintenance. You may have to lube the belt periodically. You may have to change the belt after many years of use and abuse, so it is a good thing if you can have it replaced for free under the warranty. It is important to follow the manufacturer recommendation on maintenance.