Does it support (or are there any others you've encountered that support) non-hierarchical links? A severe limitation of most mind-mapping software is that every node needs to be in its own little slot in the hierarchy; I don't think like this. I think in graphs, and interrelations – I suspect most people do. If I can't connect widely spaced nodes with some kind of edge, it's not a map of the mind at all.
Exactly my issue with all mind mapping apps I've tried - they are trees, not graphs, and I don't think that way.
I find them useful for to-do lists, but little else. Even if you can connect two separate nodes, the tool doesn't use that new information to change the organization of the nodes (something like a force-directed graph could be interesting).
Scapple just lets you draw nodes and edges. I fell in love with it a while ago and use it to collect my thoughts, and to put together project plans (treating it like a dependency tree, even though it doesn't impose that constraint on you)
