You want a case with good airflow and depending on what your using it for, you will want it to be light. Have adaquite space for all hardware and a good clean airflow. Also enough room for atleast 3 case fans, best would be 2 in the rear and 1 in the front if you have a case that is 3 fans.
If you find one that is more then 3 fans that is even better. The idea is to keep things cool, though more fans equals more dust so you will want to put a peice of fabric, cant remember the name of the stuff, kinda like polyester filling, infront of the intake fans to collect dust.
-Ryan