This is... interesting. You'll notice how the lower torso has the heaviest amount of armor on it; this is probably because the necessary range of movement for the legs is relatively limited except for around the hips - where the armor is naturally lighter.
What they've done with this armor is trade off the image of being a walking tank for the sake of increased mobility, so it's a little bit like actual armor, just more exaggerated in some ways. This armor would allow Chief to contort his torso in a lot of ways without his movement being impeded, even though it offers a lower threshhold of protection.
They say that this will reflect in the gameplay of Halo 4. If I had to take a guess, and I do, this will probably translate into more movement options. Master Chief still weighs a thousand pounds in this armor, but the freedom of movement afforded his body by the reduced coverage should allow for more varied and more detailed movement options. A sprint button, maybe? A carefully crafted cover system? Recharging shields but no recharging health? The ability to leap to the side as a standard ability instead of just something Elites can do?
It's a bit shocking the first time you see it, but the gameplay options it suggests are potentially very interesting.







