Size and cooling.
First, don't pick a computer case that is too large for you. Otherwise you'll just take up a lot of space and never use it and it will be harder to move and get around with.
Second, Cooling = Air flow. Make sure there is a good arrangement of fans and air flow in the system. No large ribbon cables obstructing. This also means good construction. Plastic cases insulate heat more then metal cases, bad screws and stamped metal edged are a cut hand away from frustrating you.
As with many things in life, you usually get what you pay for. A good case will cost you a little more but can be used again and again..
Think about it.
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The 3 factors are size, not to big and not to small, cooling, try to have cooling holes and lastly type of bag do you want a backpack a sleeve or well anything