The real downside for me is that my notes are on their server. I really hate that. So .. I keep getting back to org mode .. which isn't available everywhere.
After almost 4 years, my app is currently used by less than a few hundred people so I never thought there is a big demand in this area. I just made it for myself. It'll take me some time to re-code it (and most certainly open-source it) but if enough people like what it does and how it works, I'm all up for it.
The closest I've come is to use org-mode in emacs with the foldout package enabled [1]. This works well actually and I even wrote my thesis this way (which I can't do using Workflowy btw ;) .. but you got to manage the syncing etc yourself.
It is valid to argue that a tree structure is not the ideal mind-map, but the speed advantage of editing a tree structure way more than compensates for a more complex graph structure ... and you can always hyperlink.
[1]: http://lists.gnu.org/archive/html/emacs-orgmode/2007-10/msg0...