The first and foremost important thing to consider before looking for a desktop case is that you know the form factor of your motherboard. If you know it, very well. However, if you don't know it, you should refer to the specifications of your motherboard or check it online. It is usually of the form ATX or micro ATX or BTX or micro BTX. This is important because the form factor decides the shape and size of the motherboard and we would obviously like to buy a cabinet, that can house our motherboard.
Secondly, it is a must that you buy a cabinet that has ample space inside. In simple words, buy a large cabinet. It promotes air flow and will keep your system cool. Thirdly, the case should not conduct electricity. Ensure that the case is not metallic and that each and every screw-hold for the components has rubber or plastic insides. This will help in preventing your system components from getting an electric shock.
Fourthly, there must be ample additional cooling fan attachment space in the cabinet. This is the first and the most critical point for gaming systems. Apart from the mandatory cooling fans present in your SMPS, over the CPU, and above the GPU, there should be provisions to attach more fans at the rear, the left side and in hardcore systems, the front as well.
Fifthly, see that the case has ample of hard drive and optical drive bays, so that you can attach as many HDD's and ODD's as you want. And optionally, look out for the optional components like headphone jacks, the number of USB connectors, etc. that help you in being more productive.
The image on the left shows a nice cabinet, with good cooling provisions.