After our recent article that discussed some confusion Windows switchers might have within OS X, another key difference that a few people have mentioned is the difference between the location of application menus in OS X and Windows.
The difference is simple: While Windows includes menus in each application Window, OS X places all application menus in the system menu bar, though some applications have document-specific menus that appear in a window's tool bar (e.g., the font and formatting options in TextEdit windows). While this difference is basic enough and pretty straightforward, a few Windows users (and some Mac users) might still enjoy having an option to access the application's menu items from within the application window.
There are several instances where this might be useful. The first is if you have a background application that you would like to quickly bring to the front and perform some menu operations, and another is if you have a multiple-monitor setup, since the menu bar will only display on the main monitor in OS X.
The default option in OS X is to click the application (or use Command-Tab) to bring it into focus, and then locate and access the menus; however, if you install the program DejaMenu (freeware) you can set a hotkey combination that when pressed will display the current application's menu options at the point of the mouse.
DejaMenu itself is limited to using a hotkey sequence of your choosing, but if you combine it with some third-party mouse drivers or add-ons such as USB Overdrive, Better Touch Tool, or jitouch that support binding key sequences to mouse buttons, you can enable this for a specific mouse key and enable the menu on-the-fly.