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.
What really sucks about the Debian way is how it tries to start daemons in the post-install scripts and if that fails (say because the default config tries to use a port already taken) the entire package system shits itself and is unusable until you fix it.