A long time ago, some windowing system (I don't even remember which one--maybe someone can help?) mandated that window controls be in the upper-left, EXCEPT for the close button, which would be all by itself at the upper right.
The rationale was that it makes it much more difficult for a minor mouse slip to accidentally kill an application.
This always made sense to me, but I have to admit after years of using cram-them-all-to-the-right interfaces, I can't think of a single time I ever made this mistake.
Canonical's decision seems to be a deviation from a widely-accepted and familiar convention without even the notional advantage of the approach I described above.