Since version 11.10 Ubuntu has a feature called Global Menu. This means, that window menus don't appear in the window itself anymore but on the very top of the screen in the so called panel. Have a look at the video to see it in action:
I have the impression that Ubuntu's UX designers are trying to mimic Mac OS X here. However, there are several problems with the solution they came up with. The Mac shares some of them, but some are specific to Unity.
The Global Menu wants to give more vertical screen real-estate to the running applications by having only one menu bar taking up space for all windows together. On netbooks this may be a usefull thing, but on computers with HD everywhere nowadays this actually makes usability worse. The Global Menu makes you "travel" large distances with the mouse from the window to the upper border, which can contribute to repetive strain injury.
Another problem is that the menus are not visually connected to the window or application they belong to. For first time users it can be hard to grasp that the menu even belongs to the window or application that is currently in focus. On first sight it looks as if the menu contains global functions. OS X and Ubuntu both try to fix this problem by showing the name of the application at the left most position.
However, Ubuntu has another big usability issue adding to the previous point. It hides the menu and only shows it when the user hovers over it with the mouse. First time users will probably have a *very* hard time even finding the menu at all. In my opinion this is the biggest glitch of them, because you can't find the menu, without knowing that it is there in the first place and moving the mouse up there. I think this is called a vicious cycle.
To make things worse, I don't even see *any* reason why one would hide the menu, as there is oviously enough space to display it all the time.