The look of your machine! Every system builder usually wants to obtain the sexiest case to show off at LAN parties, and will look nice sitting next to some big screen monitor and their Xbox 360. There are a few things to keep in mind when purchasing a case other than its looks of coarse:
Key Case Factors:
2. Form Factor and Slots
3. Air Flow
5. Noise Level
This is more important than you think. My first build in 2004 had a steal case but a plastic front panel, which broke off of its plastic hinges in a few months. It looked cool from the pictures on Newegg.com, but ultimately I should've gone with my original choice: a Thermaltake case with an all steal build.
It is obviously important to have a case that your Motherboard will fit into, so ensure that if you have an ATX board you are buying a tower (aka Case) that is ATX compliant.
Also ensure that your case will hold however many disc drives, hard drives, and other accessories you plan to include in your rig. DVD/CD Drives are typically 5.25" slots, and Hard Drives are 3.5" slots. There are also other accessories, such as fan controllers which control case fan speeds that fit into 5.25" slots. For example: if you want a DVD drive, Fan Controller, and one Hard Drive make sure your case has two 5.25" slots and one 3.5" slots.
Keeping your Rig cooler is vital, especially when overclocking. Gaming on a computer pushes the components to their max, generating a lot of heat. It is crucial to have enough case fans and ventilation to expel the hot air.
Most cases builders use for a gaming rig are "mid-towers" and "full-towers." The "full-tower" offer more space, which equals more ventilation, and more slots for more hard drives and disc drives. Mid towers a little smaller, but still have room for multiple hard drives at the cost of being more cramped. This is also important to note as some graphics cards are longer than normal, and some CPU fans are huge and simply won't fit in a case. It is important to search the internet for somebody with a similar build to ensure everything will fit without having to modify the case a little (more likely than not, many people will have the same build as you do).
This is a trade off with Air Flow. To increase air flow, more fans will be needed, and they will need to be running faster to push and pull that hot air out of your system. However, this is much louder. Some people hate how loud their rig becomes, others don't mind, especially if they're wearing noise cancelling headphones. This is simply a quality that differs from user to user.