Impressive. Great job Bret!
There’s a few quirks and state errors here and there (i.e. insert functionality in spreadsheet theme) but I’d say the biggest omission and growth blocker might be lack of proper import facilities. A gdocs import, digital locker (dropbox, onedrive, gdrive etc.) importer, Asana and Trello munchers, among others, would make Quip a better case for busy people; or those who want to give it a try but don’t really feel like copy-pasting existing work into Quip.