In the books the Spartans has unbelievably fast reaction times. They were able to occasionaly dodge bullets. What if in H2 you were able to dodge bullets when your adrenaline spiked. This would be bad ass.
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I agree with mrsmiley. Dual weapons, sprinting, and more stealth combat are all new to Halo. This does have to change how you play only allow you more choices. Anything Bungie add is going to be awesome wether they give you the choice to use it or not.