[adriano_f]

Lots of interesting points, but I disagree with the logic that "we are not designed to handle more than one layer at a time" necessarily implies that a tree-editor is impossible/undesirable.

Yes, we are only built to think of an idea/code/story/essay at one "level" at a time. You are either thinking about the overall structure of the code, or a particular line. A particular scene, or the overall plot.

But that's precisely why a tree editor, one that separates the levels for you, is so useful. You can always "zoom in" to the details by going right, and "zoom out" to the big picture by going left.

Have a look at Gingko, for our approach to solving this problem: http://gingkoapp.com