With the release of OS X Mountain Lion in 2012, Apple brought iOS's Notification Center feature to the Mac. As with its iOS counterpart, OS X's Notification Center alerts you when something is happening (or has happened) on your device. On iOS it's flawed but indispensable; in Mountain Lion I've found it to be eminently dispensable.
In recent years, Apple has shown a predilection for improving operating-system features in their second iterations--think Time Machine and FileVault, to name just two. With OS X Mavericks, Apple is doing the same for Notification Center. But while the result is more useful, it still doesn't feel as organized as it should be.
Notification Center comes in two forms: Floating notification bubbles that appear on your screen when you need to know something, and a notifications list that slides out from the right side of your screen when you click the Notifications icon at the far right side of the menu bar. (The bubbles come in two forms: Banners and Alerts. Banners eventually fade away if you ignore them, while Alerts stay on screen until you act on them.) In a year of using Mountain Lion, I have come to appreciate the floating bubbles, but I've rarely, if ever, used the sidebar list.
In Mavericks, those bubbles have become much more useful, mainly because they're now more interactive. When you receive a message in Messages, for example, you can reply directly from the bubble: Click on the Reply button, and the bubble expands to reveal a text area. Type a reply and press Return to send it.
There's similar functionality when it comes to Mail alerts: You can reply directly from the notification bubble, or opt to delete the message you've just received. Deleting messages from a notification bubble might seem a bit extreme, but if you've set Mail to alert you every time you get an email, there will probably be some spam you want to get rid of in there. It would be nice if you could flag or archive messages from the bubble, too, but Mavericks doesn't offer those options.
Incoming FaceTime calls are likewise heralded by a notification bubble. You can answer or decline from the bubble, or even reply with an iMessage explaining why you don't want to take the call right now.
Since Mavericks includes the option to automatically update your Mac App Store apps, Notification Center also now alerts you about apps that have been updated, and warns you if an update requires you to quit an app or restart your system. There's also a Later button that allows you to set when Notification Center will bug you again about installing that update.
Some other system alerts have moved into Notification Center, too: Warnings about a dying battery, a failed Time Machine backup, and ejected disks that would previously have spawned a big warning dialog box instead create a Notification Center bubble.
I set my Mac to automatically lock when I put it to sleep or after a certain amount of inactivity. When I wake the Mac up but before I log in, Notification Center will display some basic information about things that have happened since I went away, including any email alerts I've received.
This might be handy, but it also seems like a security flaw, since someone could view potentially sensitive information without entering a password on my Mac. Fortunately, settings in the Notifications system preference pane in Mavericks can address this: App by app, you can decide which notifications display when your Mac is locked. For Mail and Messages, you can separately opt to show or not to show the contents of the message you're being notified about. It's great that these features exist, though I'd prefer an option that would globally prevent such notifications so I didn't have to go through a list of all my apps and uncheck the boxes one by one.
The semi-hidden Do Not Disturb feature introduced in Mountain Lion (if you scrolled all the way up in the Notification Center sidebar, you found an option to silence warnings until the next day) has been fleshed out in Mavericks. A new Do Not Disturb setting in the Notifications preference pane in System Preferences lets you mute notifications at particular times, as well as when your display is sleeping or when your computer is attached to an external display. You can also squelch FaceTime alerts using this preference.
As I say, I don't find myself using the Notification Center sidebar very often; it's usually full of lots of old junk and gives me no way to filter or quickly clear it. I wish it was better organized and took a broader overview of what was going on (a la the forthcoming Today view in iOS 7). It provides too much information, and I find myself accidentally triggering it via an inadvertent trackpad swipe more often than purposefully opening it.
Those complaints aside, one thing I do like in the new Notification Center is the ability to send instant messages from within it (not just Twitter and Facebook posts, which you could do in Mountain Lion). Just as you can reply to a message you receive directly from within the notification bubble, you can start a conversation with a friend by clicking on the Messages icon at the top of Notification Center. This seems like a great idea, especially for people who don't keep Messages open all the time.