Twitter is simple only at the UI level. It’s still very easy to make a seemingly simple move on Twitter which results in ending your career, signing up for a life of harassment, etc. In a way, Twitter hides too much of the complexity of the world.

What’s an example of this?

Jon Ronson’s book “So you’ve been Internet shamed.”

Git is pretty simple. https://github.com/git/git/blob/ecbdaf0899161c067986e9d9d564...

Git is one of those rare products whose internal implementation is simple but whose user interface is a complicated mess.

I think this is close to a pretty good definition, but it should be restricted to people who already have learned the mental models of the domain. In that the mental models learned to use the software should match those of the domain.