Notification Center for non Mac App Store apps
A lot has been said about Notification Center over the past few days, and the recurring theme has been that its potential is severely limited by "the fact that it only supports Mac App Store apps". I put this part in parentheses because I think it's simply not true.
Basically, I think that the only kind of notifications that are limited to Mac App Store apps are push notifications, in other words: the notifications that go through Apple's servers. But I think that Notification Center also allows any app to display local notifications. This is akin to how Growl works, there isn't some kind of Growl server as far as I'm aware. An app that demonstrates this is Hiss. Even though it isn't on the Mac App Store, it still does its job of sending Growl notifications to the Notification Center instead. Link this to the fact that, unlike iOS apps, Mac apps are free to run in the background and maintain their own connection to the internet, and it's easy to see that any Mac app can take advantage of Notification Center. The implementation shouldn't even be radically different from how Growl works.
Disclaimer: I'm not a developer, but I did talk to one of my colleagues who is an iOS developer (so slightly more knowledgable about the matter) earlier today and he seemed to think the same thing. It would be fantastic if a real Mac dev could step in and tell me whether or not I'm right.