aluminum- expensive, .5 db of noise vibrations, better heat disapation, looks nicer, thin(easier to mod)
steel- cheap, less heat dissapation than aluminum, contains noise better, thick(harder to mod)
look for ones that have lots of spots for fans, make sure they are big holes (92mm-120mm) bigger fans push more air with less noise. Also if the case has a precut blowhole thats a big plus.
get one with a removable mobo tray (you'll be pissed if you dont have one)
make sure it doesn't come with a power supply, get your own (never skimp on a power supply.. even in a budget pc you should spend at least $50 on a power supply) also if the case doesn't have its own psu, thats a sign that it is a very high quality high end case.
Dont get a tacky one with front cc tubes and a blazing blue front intake turbine.. those get old fast.. get a more stylish one... most cases made by coolermaster and lian li are good examples of being elegant
The more drive bays the better.. you'll never know what kinds of stuff you'll add later on.
Front ports... they are important but make sure they have covers or they will look like ass
All i can think of for now...