If you're going to clone MacOS button layouts you should be cloning the right version, namely <= OS9. The MacOS X button layout never made any sense and is a clear usability regression. Color blind users have a harder time distinguishing between the buttons. Initially the buttons had no iconography apart from the color until mouseover, when a small icon appeared. These points are sufficient to illustrate the fact that MacOS does not hold some special usability knowledge and is not automatically worth copying.

That said, cloning any Mac button layout is not ideal.

The "dangerous" close button should be isolated from other buttons, or visually very distinct. MacOS 9 and below did this by putting it on the left, by itself. Windows, recently, has done it by changing the size and shape of the button.

Having any buttons on the left-hand side is a problem as long as the menus for the application are also on the left. In classic MacOS this was less of an issue because maximized windows were not the norm and the menu bar was disconnected from the application. If the menus are on the left and the menu bar is in the application window and the close button is on the left then there is a probability that close will be clicked by mistake. Having "less-dangerous" buttons on the left is also problematic in this scenario, but not as catastrophic. Since Ubuntu uses in-window menus it is not advisable to have buttons on the left at all.

Intuition is a hard thing to pin down. It is my perspective that computer UIs are never intuitive and must always be learned; thus, there is an advantage to not requiring unlearning and relearning but no advantage to making the initial learning "intuitive." Once something is learned the most intuitive type of UI will be similar to the learned UI, regardless of what the learned UI looks like. Given that appealing to Windows users is a stated goal of Ubuntu keeping the window control buttons in the learned location makes sense.

Whatever goal might exist with regards to moving the default button location it should be remembered that this is not a design decision but a user preference. Users can, and will, change it back. When they do it is important that the experience remain pleasant; the theme must adapt to the button location and not appear 'ugly' as a result. Further, whatever is eventually meant to occupy the right hand side of the title bar must be equally able to occupy another place on the title bar, or even be absent, depending on where the user has chosen to place his buttons.

If a change were to be made then the ideal placement for window control widgets would be the bottom left of the window, not the top at all. When dragging and resizing a window the titlebar and right-hand side are the most common places people place their pointers. By putting the buttons, especially close, at the far opposite end of the window it becomes a very deliberate operation to access them. In addition, typically nothing else is "nearby" the lower left corner of a window on an Ubuntu desktop (on Windows this would still be an issue since the Start menu is there for maximized windows.) This is the least dangerous position for close and a not unthinkable position for the other controls--though I would put the Max Size toggle in the upper-left (with Sticky if present) and the Iconify button in the upper-right. The lower-right ought to be empty or reserved for a large surface area window resize handle.