Ms. Komando covers most of what a person should look for but forgot to cover the different resolution. A lot of lower priced units do not produce a high enough resolution to be considered high-def. They will display the same image, but will do so by lowering the resolution of the signal to its native resolution. You can find good inexpensive projectors for under a thousand that does have a good resolution however; just stay away from the 800x600 units if you want to use it for HDTV.
Check out Kim's guide to projectors if you have any questions about these mini-giants of TVs.