The undo/redo framework uses a stack to store all steps.
One good enhancement would be to have the possibility to store the steps
into a tree.
Vim 7 (http://www.vim.org) use a tree and this permits:

• normal undo/redo
• jump to a specific date

The good point of an undo tree is that you never loose your changes (but
costs more memory) even is you do some undo then modify the result.

The full description of the undo mechanism of vim is available here:
http://vimdoc.sourceforge.net/htmldoc/usr_32.html