Light output, resolution, contrast, quality of lens, whether or not lens allows for manual adjustments for tilt controls etc. Sure theres others but I am not a projector genius.
It comes down to a couple things really;
What your using it for
How it will be mounted and projected and distance, ammount of light in the room etc
How anal you are
If you want to use it for media content like movies and TV, your goign to have a hard time finding anything for under ~700 bucks thats going to look even somewhat decent. Also IMO and maybe im just anal but for any Big TV and especially in a projector, 720 is a waste