Not only is the D600 yielding a sharper photo, but it's also lower in noise. Why does this make a difference? Well, it really depends on how trained your eye is and whether or not you're printing your photos. When you look at your photos on a screen, are they at 100%? Or, are you zoomed out? That'll make a difference too. If you're printing photos like this, you can be sure you will see that noise. Guaranteed.
As a general rule, most people don't notice it. But if you look close in certain photos, you'll start to see it more and more. Sports photography has a lot of noisy photos because the lighting in stadiums isn't really that great. Look at your next issue of Sports Illustrated. Rummage through the photos in the articles. You'll see that those photos are noisy. It's just a fact of life.
I have two shoots scheduled in the next few days. One of which is the Mobile Summit at Notre Dame tomorrow. This will be a great trial weekend for the D600. But I have a feeling it's going to perform great. Hope to see you there!