Those changes include two new levels not found on Wii, a "new baby mutant type" and a new crossbow weapon. In addition, all of the boss fights have been "improved," and the "Mother" boss fight has been extended. Even the cutscenes have been redone with new motion capture.
There are also a couple of new modes: "Hardcore Mode," which forces you to perform headshots to kill enemies, and "Classic Mode," which gives you only the AMS Magnum. Overkill's Extended Cut will reach into retail in North America on October 25.