Obviously, a can of worms, especially when you start considering the API. :-)

Just the fact that there is some level of granularity on the Tiddler end means that you can support some form of real-time editing on the client-side. For example, if there were a Trello-like front-end, different people could be working on different cards / Tiddlers at the same time (which is how Trello works right now).

Still, the lack of real-time editing means that I have to use something like Google Docs or Etherpad for real-time, then migrate it to a wiki. Hackpad isn't the greatest wiki, but it's real-time editing makes it fairly compelling.

Furthermore, I find the "Save" button frustrating. :-)