What is the extra $1000 in the new EF 24-70 going towards?

Well first of all, it's to offset Greece and the Federal Reserve.
Other than that..

The Old 24-70

The New 24-70

That said, I really wish they'd have kept the reverse-extending design. I thought that the more efficient lens hood would be the way forward with all mid-range zooms.
Personally, I don't care for IS at this focal length. It's my prediction that in a couple of years, few will care for IS at all unless they're shooting a supertelephoto. IS will be great for stabilizing your VF and not much more.
When you can get clean images at ISO 102k, you will be happy that Canon put all that size and weight into IQ rather than a crutch.