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.
And know how to use an existing btrfs partition. And always [at least have an option to] show exactly what the automatic installer is going to do before I run anything. There's gotta be a middle ground between "we'll just surprise you" and "here, do everything yourself".