I think some of these replies have perhaps missed the powerful idea that made me fall in love with Eelco Dolstra's idea. Here's what won me over.

For example: THE main feature is that you could have a different version of say Python (for the sake of this example) installed for each dependency in your system. Let's say you had Brave working with one version of Python and another piece of software needed a previous version of Python. In an FHS style system, this would be challenging and you'd have to manually patch things to make sure the dependencies didn't step on eachother. When you updated, your patches would likely have to be changed as well. So, system administration and updating can really break things.

In a Nix store where things can be content-addressed and linked by symlinks to their specific dependencies, they would just work alongside each other due to their unique, hash based folder locations. Each folder in the Nix store is named based on the sha256 hash of that piece of software's ENTIRE dependency graph, which has powerful implications.

Because of this hash, they’re effectively hermetically sealed from each other and cannot step on each other. The software in the Nix store talks to eachother through symlimks that were made upon compilation of the system.

This is the very definition of Nix and taken far enough to define a whole OS is SUPER powerful concept.