In broad terms, the new Xbox 360 "S" model is the same as the previous console; it plays the same games, does the same stuff. But under the hood, there have been changes, and mostly for the better.
AnandTech have put the new Xbox to the test, comparing it to three previous iterations of the 360 to see how it stacked up in terms of power consumption and noise. You can see the results below. In short, it uses a lot less power, and is a bit quieter as well ("noticeably" quieter, according to AnandTech), spinning game discs at 51 dB(A) instead of 54 dB(A).