It does emit the signal when files are added to or removed from the directory. It does not fire the signal when existing files are modified. According to the documentation: http://doc.trolltech.com/4.6/qfilesystemwatcher.html#directoryChanged it should:

This signal is emitted when the directory at a specified path, is modified (e.g., when a file is added, modified or deleted) or removed from disk.

This can be fixed by modifying qfilesystemwatcher_win.cpp:QWindowsFileSystemWatcherEngine::addPaths so that the flag

FILE_NOTIFY_CHANGE_LAST_WRITE

is added to the flags for directories as well as files so that the code becomes:

       uint flags = isDir
                           ? (FILE_NOTIFY_CHANGE_DIR_NAME
                              | FILE_NOTIFY_CHANGE_FILE_NAME
                              | FILE_NOTIFY_CHANGE_LAST_WRITE)
                           : (FILE_NOTIFY_CHANGE_DIR_NAME
                              | FILE_NOTIFY_CHANGE_FILE_NAME
                              | FILE_NOTIFY_CHANGE_ATTRIBUTES
                              | FILE_NOTIFY_CHANGE_SIZE
                              | FILE_NOTIFY_CHANGE_LAST_WRITE
                              | FILE_NOTIFY_CHANGE_SECURITY);

I don't work on the other platforms I can't compare this behaviour to what they do.