On Mac OS X, the modifier keys map as follows:
S+Shift
A+Option
C+Control
M+Command (Apple)

On Windows and X11:
S+Shift
A+Alt
C+Control
M+Windows key (often called Super on Unix)

On Windows, key-up gestures are not reported for all keyboard events.

Multi-touch gestures are only supported on Mac OS X.

File drop gestures are only supported on Windows.