The biggest thing to consider is airflow. Poor intake will limit the amount of cooler air coming into a case, which helps keep not only the CPU and chipset cool, but the power regulation circuitry as well. Cooler components tend to last longer than hot ones. The reason I say this is that there are some cases that have very poor front intake, and while they might look sweet to some, unless you're absolutely dead set on having it, look elsewhere.
Secondly would be your use for the system. If you're going to be going to LAN parties, move regularly, or just need something small, a smaller case is in order, made out of light steel or aluminum. My main box is probably 30+ pounds, so it's a real treat when I have to move it around. As you would expect, a smaller case is generally lighter than a larger case. The advantage of a larger case lies in more room to work and the greater likelihood of more drive accomodations.
Third, the sturdiness is also important, especially if this is going to be a fairly mobile box.
