I did a ton of research on treadmills last year, tried out a bunch and ended up buying a True ES900 for about $4500. Probably more than I needed and sounds like more than you want to spend but here are my thoughts.
1) Think about what features you want. Do you want no frills - just something that has multiple speeds and multiple inclines? Do you want decline? Do you want adjustable decks that can be made to feel softer/harder? Do you want multimedia - i.e. internet connection, tv, music when you run? You pay more for the latter options and generally pay more the more options the treadmill has. But be warned that like cars electronic gadgets are more likely to malfunction.
2) Conventional advice I was given is that $2000 is the entry price for a solidly built treadmill that will last and hold up to wear and tear. Biggest contributors to quality are the motor and materials. So look at motor specs and whether materials are metal or cheaper plastic. Icon fitness - makers of Nautictrak and proform - are the largest makers in the industry but I was warned to stay away because they are known for crappy materials and poor service. These treadmills are more cost effective because of the scale of the manufacturer so consider buying one maybe with extended warranty if cost is an issue.
3) Run on them if you can. I was shocked how differently each treadmill felt when I ran on them. Some shimmied at higher speeds and some felt too soft/too hard. Some were difficult to adjust speeds and inclines quickly. At the end of the day you run on the thing so make sure it works for you. If you race on hilly courses consider decline option.
4) Check out reviews at treadmilldoctor.com
I bought mine at GymSource - which is all over the Northeast I believe.