Image quality is the most important for me. When I buy a camera I read reviews to make sure the camera delivers good quality images. After that I'll look at MP rating, optical zoom, features and method of storage.
Personally, because I shoot sports for a living, I need speed. I will look at shutter lag, burst rate, buffer size, write speed, viewfinder blackout and AF speed first. Next is how responsive, intuitive, and customizable the features are. Image quality is next (color fidelity, noise, dynamic range, etc.), then construction (will it hold up to rigorous field use and abuse). Last, but not least, image resolution.