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.
In these events they talk about the new features as they're the most recognisable for press, often examples of what some of the new APIs are capable. (Features, which could of course have been programmed to run atop 10.7 Lion.)
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.
Don't forget all the API changes that present themselves with each new major version. For example, Notifications, GameCenter and Twitter are all new core libraries on the Mac that are accessible to developers.
What they cover for the general public is usually only the most visible features. There's additional new stuff under the hood, which will be available for 3rd party developers to use in their apps.
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.
This is an early preview. We don't know about all the new APIs and changes under the hood. Presumably there is a lot going on with the additional iCloud support.
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.
