The difference is how the signal is sent. When you change the resolution in XP for instance, a low res picture gets sent to the display which then has to up sam

The difference is how the signal is sent. When you change the resolution in XP for instance, a low res picture gets sent to the display which then has to up sample it, and it looks sucky. On the new MBP the picture sent is always the native resolution of the monitor, you just change the size of the UI elements according to your preference.