Speaking about Gentoo, you don't "have to" compile everything, but let's just say it's strongly encouraged. Most of the compilation is automated though, think AUR on steroids. If you don't want to touch anything, you can go with defaults for all the software and you'll get basically what you get on any other distro.

The thing is, though, if you are not being a little funny with your software, why the hell are you running Gentoo? So, it's entirely optional, but it's the main reason one would use Gentoo, otherwise you get a stock standard system with stupidly long updates.

I can't speak for Nix, though, I haven't touched it myself, but from what I know from those around me running it, it's pretty much the same thing, unless you want something cool, you don't have to suffer.

Not really. (When was last time?)

The reason is more that the config language that you have to learn if you want to use NixOS is the exact same you use to package applications for NixOS.

So any user is also just a tiny, non-intimidating step away from being a contributor and package maintainer.

I know for myself, I never packaged for the AUR - just seemed way to intimidating and a lot of new stuff to figure out - but I maintain a bunch of packages and modules in nixpkgs, and they're all stuff that was "huh, this is cool but not packaged yet; alright, I'll package it in my private config; oh that was easy, let's upstream it".