One major headline this time is the new focus on performance. True 64-bit support and Adobe's Mercury Performance System play a big part in improving responsiveness, but Adobe have also been identifying and resolving bottlenecks all around the package. The previous Gaussian blur implementation wasn't the best, for instance, but it's been optimised for CS6, and as a result Gaussian blurs (and effects which depend on them, such as drop shadows and glows) are now much, much faster.
It's a similar story with the interface. There's an obvious new look (you can even tweak the UI brightness to your personal taste), but Adobe report making more than 40 usability tweaks to the Illustrator interface, from easier font changes to inline editing of layer names.
Please note, the file you'll download here is Adobe's Download Manager, which will then fetch and install Illustrator for you. It's a huge file, though, so patience will probably be required.
Download: Adobe Illustrator CS6 v16.0.3