The ring around the front sight is replaced with vertical "ears," the rear sight has open slots instead of apertures, the magazine release was made wider, a Picatinny rail was added to the top of the receiver, the pistol grip was given a stippled texture like a Glock RTF frame, and a sling attachment loop was added to the front of the trigger guard. Appearance-wise, the "cocking tube," charging handle, and selector switch have changed.
Christopher at ATI says that parts from the original GSG-5 will be available to those who want more of an MP-5 clone, but that the manufacturers, distributors, and dealers can't sell them that way.
Also new is a 110 round drum magazine!
You can read a little more about the new GSG guns, and see all the pictures, in this AR-15.com Forum thread.