This is possibly going to be the smallest "What's New" segment I have ever written. To put it plainly, the only thing new with these CPUs is the addition of the new overclocking features.
Now, I know you are wondering (like I was) what you get with these new features. Well, you now have the ability to control the CPU multiplier. You can adjust it higher than the stock settings.
For example, the i7 870 normally has a multiplier of 22, but with the i7 875K you can push this to over 22 (we used 24 and 25 at different times) to get the highest clock.
Intel has also added in a few more options for memory dividers; again to give you more flexibility for overclocking. Finally, they mention that there is an increased tolerance for voltage (they say current and power).
Everything else is pretty much the same. The i7 875K is still 45nm, the i5 655K is still 32nm; Intel is just giving you more room to push these beyond their normal operating speeds.
See, not much new there. But we are hoping to see quite a bit more performance out of the i7 875 when we push it.