I’m not going to dwell on the subject – The Ubuntu Design team (and in the end Mark Shuttleworth) decided to change the titlebar button layout as part of the new Light GNOME theme (which is a really nice theme); for more information read this article
Being the good Ubuntu fanboy that I am, I have really tried to embrace this new layout – But every time I move the mouse pointer to the right side of the window only to find the close button missing, I grew increasingly frustrated.
Luckily the fix is easy:
From a terminal (or from the run prompt, ALT-F2), run
gconftool-2 --set /apps/metacity/general/button_layout --type string menu:minimize,maximize,close