An app that I posted about just over a week ago is now out of beta and
in the Mac App Store - FoldingText is available for $14.99. It's currently at a 40% off launch sale1.
Like I said in my original post, it won't be for everyone but it could hit the sweet spot for a few in the note taking, to do list/GTD area.
Even at it's sale price of $14.99 it's a bit too much for what I can currently see myself using it for.