You should check out the Norco cases. They're 4U rackmount units with 24 hot-swap HDD bays, designed to fit regular ATX motherboards. Very cheap for what you get (also include a ton of fans). The build quality is not spectacular, but for a home system where you're not doing a whole lot of swapping they're great.
^^ This. Having hotswap also makes a big difference when you start having lots of drives. I'm still using a Norco RPC-4020 from about 3 years ago, 20x hotswap bays.. with the backplane mounted (8cm) fans running at 5V. Makes it acceptably quiet.. a lot of the server grade hardware will be set to tornado mode by default, as who cares about noise in a datacentre.