Google has been testing Jazz since November at least - they must feel that they've got an improvement over their traditional interface or else it wouldn't be rolling out.
I think the main idea is to surface those left side options and get more usage for all that functionality. That left side toolbar could make a lot of difference. It will expose more URLs to more people, and wreak some havoc with ranking reports and predicted CTR at the very least. I'd guess that at least some of the strangeness in the new WMT reports [webmasterworld.com] ties into the Jazz interface.
It's interesting how easy it is not to like any change. I remember having an immediate negative response when I first saw the current Google favicon, for instance. Rather quickly, I "got it" but at first - no way.
Tedster: I do not oppose, I just wonder about the little attention this got over all. When all SEO is done, the interface is your next step of work. So professional SEO work strongly involves interface design and I expected to see more rants/ideas/praise of these changes now that they become more and more visible...