For example, I'm using Debian, and I think we could learn a thing or two from Mint about how to make it "friendlier" for new users. I often see Mint recommended to new users, but rarely Debian, which has a goal to be "the universal operating system".
I also think we could learn website design from.. looks at notes ..everyone else.
I think NixOS would stand to benefit a lot by taking inspiration from openSUSE's YaST system configuration tool. I think that if NixOS had a well supported graphical interface for creating and managing the system config, it would become so much more accessible to a very wide range of users who never would have given it a try otherwise, which in turn would bring in tons of new users and developers who will want to improve nixpkgs, etc.