my only “must have” is dual sided access. Discovered rather late in the piece that once it gets to a few hundred kilos you can’t move it, wheels or not, and having to pull equipment out the front to access the ports/power at the back is just a pita.
Only other concept is the heavier it is, the closer to the bottom it goes. Everything else is personal preference - not really such a thing as a catch all “must have”.
While I don’t have a “rack” setup, there is one essential for any NAS setup: A good UPS with a USB port. This will help with both brief power outages (less than one minute) as well as extended power outages (more than 30 minutes) for your server to gracefully shut down.
The Supermicro 1U chassis at the top isn’t in use currently. Next (at the back) is a Brocade ICX-6610-48P switch (used Brocade switches are an insanely good deal, BTW). Next, a Dell Poweredge C6220 II, the first two nodes of which are two nodes of my Proxmox cluster, along with the R630 below it. Below that is my NAS, of course. Below that is my UPS, and its external battery pack below it–which isn’t in use since I started using an external pack of LiFePO4 batteries instead. I need a taller rack.