It’s like taking the Notification Center from iOS and putting it in OS X.
Well … actually, that is what it is.
It will be interesting if Apple can somehow sync notifications between OS X and iOS if they are from the same/similar applications. That would be cool if problematic.
Also, what is going to happen to Growl? Is Apple going to give some decent APIs to work with so that things like development testing tools can hook into it? A lot of unanswered questions.