The "OS", not the computer system (hardware) still brings a lot of things to the table. Though it many ways (software) it is behind.
What drives the interest for me (not a daily OS for me or anything anymore) is the simplicity of the OS.
The concept is this. A Single User OS.
The entire OS is not "installed". It's simply in some folders. Commands in the Commands folder, Libraries in the Libraries folder, Drivers in the Drivers, etc.
The entire start up process is a Text file, that you can easily look at and modify.
How is this good?
It's easy to control your OS. Want to back up you OS? Just copy it to a thumb drive. Heck run it from a thumb drive. Run it from a network drive. There is no registry for software. All software is separate from the OS, like "Portable Apps".
Why is this bad? Probably bad for security if it ever did get popular. Harder for companies to use DRM.