1) They place unwarranted emphasis on menu bars. Menu bars are a UI element that should not be used frequently. In dealing with basically any application, most of my time is spent manipulating the content, followed by using the tool bars, then lastly menubars. In my word processor most of my time is spent typing, followed by quick formating and saving from the toolbar. In my web browser I frequently interact with the webpage, use forward and back buttons, but how often do I need to use stuff from the menu bars? Its only on the odd occasion that I need to open a website from my harddrive or get help.
Toolbars should hold the most common tasks for quick access, while menus are good because they can hold a lot of commands to all the less used operations. Menus are slow and error prone. It usually takes quite a while to find what you are looking for, and, as with any nested structure, slight movements can put you in the wrong place especially with things as narrow as menus. Menus are an important part of UIs because they allow you to do uncommon tasks, but why make uncommon tasks available at the easiest to reach spot on the screen?
2) A global menu bar is disconnected from the task at hand. It appears to be global, but functions local to what you are working on. If the focus is on the wrong window, for various reasons, it can lead to very confusing and potentially destructive behavior. This is just confusing, toolbars are attached to the application, isn't it inconsistent to not do the same with menu bars? There is no quick way to tell without thinking which application the menu is functioning for. If menus were connected directly to the task, this problem would not exist. Global menu bars were fine when most people were running one application, but in todays multitasking world it is a poor choice. Global menu bars appear to be a part of the OS by appearing globally, but confusingly function local to the application.
3) Global menu bars don't work with focus follows mouse.
4) Global menus don't work well with multiple monitors. For a given application, you have two choices. First of all, you could put the menu always on the first monitor (like OSX). This means if you are using an application on the second monitor, you have to move your mouse to the first screen to use the menu. Confusing and inefficient. The second choice is to put a separate menu on each screen. This ends up being more efficient but just as confusing.
5) This really only applies to GNU/Linux and Unix desktops, but having a global menu bar would be inconsistent. With Macs, Apple has control over the GUI. On GNU/Linux, there are dozens of toolkits floating around, and if one desktop environment switched to having menus at the top of the screen, there is no way that every toolkit would do the same. You would end up with a desktop with some applications having menus at the top of the screen, and others with them on the application.
6) Widgets must be dynamically changed. You essentially have a moving target.
If you would like to see your thoughts or experiences with technology published, please consider writing an article for OSNews.