In short, Gabe doesn’t like it. I agree with most of his critiques — Paper’s rendering engine is great (and it looks absolutely fantastic on a Retina display), but there are a few areas the app feels lacking and at times frustrating.
As far as Paper’s lack of chrome, John Gruber described it as the tension between simplicity and obviousness. In his link to Paper last week, he wrote:
Note the complete lack of persistent on-screen UI chrome — there is a fork in this regard between Apple and third-party iOS developers. Cf. Clear for another recent example.
The tension is between simplicity and obviousness. Eliminating on-screen chrome is simpler, more elegant and beautiful. But Apple’s use of minimal but persistent on-screen chrome makes things more obvious. Big differences can result from a slight shift in priorities: simple and obvious vs. obvious and simple.
I don’t mind apps that lean towards the simple approach. People often relate gestures to the keyboard shortcuts of iOS. I am a keyboard shortcut junkie, and so if an app that I use has some clever and useful gestures as a replacement of UI chrome, then I’m fine with that.
And, as Sebastiaan de With points out, after 5 years of iOS, Apple is now starting to gradually move in that direction as well. And I’m glad — the new Lock Screen camera functionality is now one of my favorite things about iOS 5.