IMHO it is a bad idea to move the maximise, minimize, close buttons to the left.
1) There is no provable usability advantage;
2) It eliminates the application icon making windows less recognizable, since before one could implicitly rely on associative visual memory, while now the explicit effort of reading the title bar is required;
3) It makes the association between windows and windows in the taskbar weaker, since before one could rely on associative memory (the application icon was preserved in presence and position between the window bar and the entry in the task bar) and now it is not anymore the case;
4) It gratuitously breaks a convention (these buttons are on the right)
5) It increases the chance of hitting one of these buttons by mistake looking for the file menu (and as already said by others to hit the window close button by mistake is not nice)...
But it is really no big deal until things can be easily configured back as they were.
Only, Ubuntu should either have their customisations accepted upstream or ship them confined in ad-hoc packages. Namely, have a gnome-ubuntu-customisation deb that when installed gives the ubuntu behavior and when not installed the upstream one. And yes, of course with this it would be perfectly ok to have it installed by default.