- Research Meetings: whether to do them over Skype, or Google Hangouts, or http://mathim.com or awwapp.com, or even with phones on a conference call.
- Audio/Video/Chat: if over skype/G+, whether to do audio, or video, or chat. And then the obligatory pre-videconference primping to look presentable (or the reliance on bandwidth failure to NOT go on video)
- Coordination: careful calculations among multiple time zones and daylight savings times to plan meetings. The use of Doodle, Google calendar, or even random emails passed around haphazardly to plan said meetings.
- Writing I: the protracted negotiations over whether to use git, svn, rcs, or cvs, or Dropbox, or random emails passed around haphazardly (and yes I've done all of this, sometimes at the same time)
- Writing II: if using an actual modern VCS, then the equally protracted negotiations over who's hosting it, who needs account access, and where public keys need to be placed, and why CS researchers in the 21st century STILL need tutorials on ssh.
- Writing III: Dealing with comments on a writeup: as fixmes, as todonotes, as issues in the git repository hosting the document, or as random emails passed around haphazardly.
And of course all the collaborator "digital" personalities that emerge irrationally and violently. Alice hates using bibtex, and Bob will only use personally crafted bibtex files. Charlie loves his own special fonts and will have raging debates over $\varepsilon$ vs $\epsilon$. Dan has never used version control and doesn't see the point. Erin handcoded her own version control system in a cutting-edge fragment of Haskell and refuses to use anything else. Frank hates online meetings, and Mallory only thinks online during a meeting. Oscar insists that Postscript is Turing-complete and is therefore sufficient for all drawing needs. Peggy insists that git rebase is Turing-complete and is therefore sufficient to fix all commit disasters... eventually.
Note to all my collaborators who I'm currently writing papers with: you are awesome and have NOTHING AT ALL to do with this list.