This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Here are some things to consider:
(1) Motorized vs. nonmotorized. If your goal is to bring your outdoor
running indoors, then a motorized treadmill is what you want. Nonmotorized
treadmills will certainly give you a workout, but they do not simulate true
outdoor running since you are driving a belt as well as your body. Many
nonmotorized treadmills only work at an incline, and pace is not constant
as on a motorized treadmill (although in this respect they are similar to
(2) Ruggedness. If you are really going to run on your treadmill, you
need something more than the $299 specials you see at various discounters.
Some things to look for: welded frame, large rollers (consider that some
club models have rollers on the order of 8 inches in diameter), large motor
(1.5 horsepower minimum, with 2.0 or up preferable). THE HEAVIER AND
FASTER YOU ARE, AND THE LONGER YOUR WORKOUTS, THE MORE IMPORTANT THESE
THINGS BECOME. Most treadmills are not built for people weighing more
than 250 pounds.
(3) WARRANTY AND QUALITY OF SERVICE ARE OF PARAMOUNT IMPORTANCE. I
wouldn't settle for anything less than a full 1-year warranty. Treadmills
are like cars; they will almost certainly need some work at some point.
(4) Maximum speed of 10 mph or more. This is 6:00 mile pace, which
will do for most people. There are treadmills which can achieve 12 mph (5:00
pace); I haven't heard of any which go faster, but they probably exist.
Personally, the consequences of a misstep while running indoors at 5:00
pace scare the hell out of me!
(5) Method of belt lubrication. Running belts can get quite warm and
wear faster if not properly lubricated. Some models are self-lubricating;
others require periodic lubrication/waxing.
(6) Ability to simulate actual running. Various mechanisms have been
developed to make treadmill running feel more natural. Without putting
in a plug for any particular manufacturer, I would recommend trying out
several different makes. It is surprising how a treadmill that feels so
natural can suddenly feel terrible after you try a different one.
(7) Manual vs. motorized height adjustment. I've used both, and I
strongly recommend motorized. If you want to run courses that simulate
real outdoor runs you don't want to be cranking a handle all the time,
especially if you're running fast.
(8) Noise level. This can vary considerably, but note that "quiet"
does not necessarily mean "better."
(9) Programmability. It should be a given that speed and incline are
adjustable during a workout. It is also very desirable to be able to
PROGRAM both speed and incline to create your own custom courses. Many
manufacturers include their own preprogrammed courses in their electronics,
but it is less common for them to give the user the ability to do this.
(10) Low price? Realistically treadmills for serious runners are going
to cost more than $1000, and they can be a lot more than this.
(11) Incline range. Most treadmills have inclines ranging from 0 to
10%. There are some which can produce a decline (-2% for example). See
below for conversion between % incline and degrees.
(12) Board and belt type. Some treadmills have shock-absorbing boards
and/or soft belts to provide a more forgiving workout than can be obtained
on hard pavement.