I've been thinking about the fact that you can't have a Paper document together with other documents in a normal folder. Because of this you need to maintain a separate folder strucutre for your Papers and invite people to them separatly.
What do you think about Paper being a sort of a document type instead of a completely separate feature (similar to how you can have both Word docs and Google Docs in a Google Drive folder)? That way you don't have to maintain a separate folder structure for your Papers, and you can share a Paper with other items simply by sharing a folder they are both in.
I can see a problem here with sharing folders as you would need to have separate settings for Paper files and other files perhaps. Opinions?