It brings the Live View from the MarkIII, bumps up the pixels to 10 million, brings over the dust reducing sensor shake from the Rebel XTi and is just something new and cool to play with.
(Who needs a MarkIII anyway? It just makes you a lazy photog and takes a large chunk of your bank account.)
Take a look at the details. (P.S. Told ya Fang, suck it!)