My concern for the long-term viability of the entire Ubuntu project is what happens once a user's system starts notifying them that they should upgrade their distribution. Not sure if that happens for non-LTS releases or not, but eventually it'll happen. The user will click 'yes' and then be presented with an extremely different windowing paradigm without having asked for it. It's like having clicked 'upgrade' to go to Lucid and finding all my window buttons on the other side for absolutely no reason. Tech folks like you and I can adapt, but will Grandma continue using Ubuntu if every 6 months something crazy changes and she has to re-learn everything? She won't--she'll get fed up and ask for her "old computer" back, which means formatting and installing Windows from the rescue CD.

I guess what I'm saying is that you can't be both a viable alternative to Windows and Mac and also be a paradigm-changing evolutionary UI experiment. The two markets--nerds and grandmas--just don't mix, at least until the next generation or two.