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.
Fedora's installer is abysmal. There's a number of installers it could learn from. They're working on one at the moment, so I hope it's good.
Enabling access to proprietary software should also install audio/video codecs. Or at least have a separate checkbox for it, like (I believe) Ubuntu has.