(Updated on 9 May 2018 for hard drives and cooler space.)
Here are my brief notes on what to think when getting a computer case:
Motherboard Size: There are quite a few options, ATX (better expansion), MicroATX (good expansion and smaller footprint), Mini-ITX (very compact), etc. Think carefully how much you will expand in the future.
Power supply Size: Either it is internal or external. For internal, we usually have ATX (more readily available) and SFX (more compact). Some small cases expect you to have external power supplies. Some motherboards make the situation easier by just requiring only 12V power connections.
Intake and Exhaust Fans: They should be balanced, or slightly biased to more intakes. (Here is why.) Note the fan sizes supported by the case. If fans are bundled, check the pins (3 / 4 / Molex). 4 pins come with PWM (pulse-width modulation) are the best. Use PWM expander if necessary. Avoid fans with Molex power supplies unless you know what you are doing. For an ATX case, I would prefer 2 to 3 intake fans and 1 to 2 exhaust fans. The difference of 1 is because the graphics card also exhaust air.
Hard Drives: How many drives do you want? Nowadays, hard drives are large and some designers assume people only have one or two of them. It is fine for a typical desktop, but not quite right if you want to experiment with lots of hard drives. Features like ZFS, Storage Spaces, and even ReadyBoost really benefit from multiple drives.
Cooler Space: Most people like to have large coolers as they look cooler (and also cooler, you know what I mean). But don’t go too big otherwise the case may not fit.
Cabling Space: Usually, system builders use the space behind the motherboard tray for cabling. The space should be wide enough (half inches) or else thick cables cannot go through. Cables can also go through the front of the tray if there is enough space after the motherboard is installed; it is exotic but possible.
LED Signal Lights: Usually, computer cases come with two lights, one for power, one for hard drive activities. Simple cases may use two-LED approach. While being old-school, the hard drive activity light is actually useful to know a system is busy loading from the storage. Avoid inexpensive cases that use gimmick blue LEDs as they do not think of the brightness.
Expansion Slot Extrusion: Check the quantity of the expansion slots. Check whether you accept the expansion slots extrude out of the case body. This is a cost-saving measure as it requires less folds. If you don’t like that, double check before you place the order.
Side Panels: A side panel can be opaque, with acrylic window, or with tempered glass window. Avoid side panels with fan holes like plaque as they will suck in dust (balanced airflow helps, but not 100%.)
Sharp Edges: Check the case for sharp edges. One infamous example is the extruding PCI bracket locks.
Materials: Some light materials such as aluminium are prone to amplify the noise inside the case. Some heavy materials like iron are prone to corrosion. Strike your balance.