It forces you to focus on content, not layout, and it has a great editing & commenting experience. Basically it's Google docs with fewer features, and then the remainder very well done. This makes it a joy to use.
Even comparing Google Docs against itself, it fails. Slow to launch, poor search, document corruption. The only good quality is being able to plus someone a task.
Paper does that.
I’ve been using Paper for months and it feels old and stodgy now when I open a Google Docs.