Plenty of space inside. I tend to like big air coolers.
Lots of fans mounts, and the ability to add any brand I like (no space issues) Preferably option to add a water cooler radiator?
Possibly the ability to turn fans on and off from a control panel?
Lots of drive bays. I have a LOT of hard drives
3.5" and 2.5" hot-swap drive bays or an e-sata port.
Fairly modest design-I don't need it super simple, but I don't want it to be super flashy. Big vents are ok, but I don't want lots of built in LEDs. Built in LED fans are fine.
I'd like to keep it around $100, but I can spend more if necessary.