is there a specific reason they needed to update the entire OS for this ? Most of the new features are actually apps, and the rest wouldn't really justify a major version change.
E.g. growl does system wide notifications, but it's not built-in, and Apple like to control these things so they may more easily interface them with other Apple devices. (E.g. Apple could very easily make their Apple TV notify the user's iPhone when a new TV episode has been downloaded, or even schedule it onto their Mac.)
With that in mind Apple have a history of meshing the experiences across their devices, the messages app is a recent example.
I hope that could be decoupled, making life easier for application authors: http://bergie.iki.fi/blog/application_quality_assurance_in_l...
The benefits might not be apparent immediately, but they'll start showing up in apps written to take advantage of 10.8's features. Some improvements in the frameworks will effect existing apps that use those frameworks.
One of these years Apple is going to discontinue carbon API support and go 64-bit only. I doubt this release is it, but those kinds of under-the-hood features are not the kinds of things that you would talk about at this point. We'll learn more at WWDC.