Not everybody wants a two foot high monster sitting beside them. Once you've decided on what size you want, there are many options within that size regarding the number of internal and external drive bays. It all depends on the components you currently have and the ones you think you might want to add down the road. You may even end up changing to a different size case, if you can't find the combination you need. Speaking of fans, cooling and ventilation is another area to consider when making a case purchase.
Many cases come with a built-in fan and room to put additional fans, if necessary.
As well, depending on what components you have, a case with good ventilation might be something to consider. Now, with the big honking power supply you've just bought and the multiple fans whirring away cooling the seven hard drives, yet another consideration you might not have thought about is noise. Like a dishwasher, computer cases vary in their noise-making abilities. Some are quieter while others whine and howl. There are other neat little computer case bits you might want to know, like the fact that, while most cases are made entirely from steel, there are lightweight aluminum cases with plastic covers now available.