Great to see another product in this space ( https://www.scigit.com/ launched not too long ago). As far as writing papers, my old-fashioned P.I. will probably never budge from Microsoft Word, so Google Docs is the best solution at the moment. The fact that they have "Google Scholar" integration for inserting citations into the editor makes it pretty much a no brainer for writing a draft.
Yikes! What field are you in? For most STEM folks, TeX seems to be pretty standard. I'd really love to see a collaborative TeX editor (web based?). Better yet... a collaborative emacs!
Rudel (http://www.emacswiki.org/emacs/Rudel) is one example of collaborative editing in emacs.