There are so many competing products in the treadmill marketplace that I found it daunting to even begin to know how to compare different models. Based on my own experience I recommend the following, keeping in mind that I use a treadmill recreationally for general fitness.

The capacity of the motor does make a difference so go for something in the continuos horsepower range of 2.5 to 3.0 HP. Another important feature is how many preset and how many manual settings the treadmill has as well as the size and quality of the display. I don't know about you but i like to see where I am on the programmable circuts and not all have this displayed.  I found at least 6 preset programs was good for me, this gave a good range of training options.

The warranty the manufacturer supplies is also important, some have lifetime warranties on the motor and frame and others don't. The width and length of the platform does also vary and for me, being 6ft, this was an important consideration as the last thing you want is to have a machine that cannot accomodate your stride size/style.

Also, pay some attention to who the manufactuer is and whether they specialize in fitness equipment, also where the unit is made. Some treadmills can have fairly scarse attention to detail, like the first one I bought which had some very flimsy plastic shroud that broke within a month. I also found the built in heart rate monitors don't work so well as your hand pressure can vary and influence the reading.

Another point is how noisy the unit is, many people put them in basements which can amplify the noise. Some treadmills boast a 'Whisper Quiet' motor. This could be worth a few extra $$. The flexibility, or shock absorbtion of the actual belt is also a very important consideration, particularly since this is something you run or walk, or both, on. If you can, try some out before you decide.

Some treadmills now have the ifit downloadable program compatibility. If you are a heavy user I imagine this feature would be great. For me, I know I just don't have the time or inclination to do the downloads and fuss with all that, so this was not a desireable feature for me.

Finally, you wont get all the information you need from the online review websites. Ask anyone you know who has one what their experience with a particular brand is. Visit a fitness store and ask about particular brands and models, these guys sometimes can give you the most useful information and insights when it comes to reputable brands, performance issues and warranty/support follow through.

In my own experience I bought for my home from a large manufacturer who sells online at a cost of $1100 - I really was not so happy with the quality and performance of the product but it is just OK. (made in China)

Later, armed with some experience, I bought a different model / manufacturer for our workplace gym at a cost of $1400 -  I was very impressed with the quality, the ergonmoics, avaialable programs and the dispay quality of this one (made in USA) and had I known when I made my first purchase what I know now, I would certainly not have chosen the model I did for my home unit.