While there's not a whole lot of groundbreaking stuff going on in this OS X release, it's still got some nice features oriented towards more demanding users of Apple's desktop operating system. First and foremost, Apple finally addressed the Mac's rather abysmal support for multiple displays. Each display can now finally have its own menubar, and the dock can be visible on any screen you desire it bo on. A fullscreen application on one, and the desktop on another is also possible (it's magic I tell you). If you have an Apple TV, you can use your HDTV as an additional display. In fact, this alone is enough reason for me to buy an Apple TV - very handy.
Mavericks also adopts actionable notifications, like the ones Android introduced. You can reply to messages, delete emails, and so on, and so forth, straight from the notifications themselves. Websites can now tap into notifications as well, so that they can send you notifications even when Safari isn't running. Quite handy.
The Mac's perpetual black sheep, the Finder, also got some love. Apple finally got around to implementing tabbed file browsing in the Finder. Apple also added tagging to the Finder and other parts of the operating system, so you can, say, tag all files related to your thesis with "thesis", and easily find them from the Finder sidebar. Why you wouldn't just use folders instead is beyond me, but alas, I've never been in sync with how Apple disregards the file system.
At the lower levels, Apple also did some interesting things. The company focussed on power use, and implemented several technologies to reduce the power consumption of the operating system: App Nap, Timer Coalescing, and Compressed Memory. Compressed Memory compresses the least recently used data residing in memory using the WKdm algorithm, which not only frees up memory but also reduces the amount of swapping going in the background. Not only is this faster than swapping to disk (even to SSDs), but Apple also claims it saves power - essentially, that compressing data in memory uses less power than writing data to disk without compressing it.
As the name implies, App Nap will limit the resources allocated to invisible applications (e.g. those hidden by other windows ) and that aren't actually doing something. They're not pausing the application; they're allocating less resources to it. This should lead to obvious power use reductions. I'm curious to see if this will cause delays when switching applications - Apple obviously claims it doesn't, but I'm sceptical.
Lastly, there's Timer Coalescing.
As a final note, the old and venerable Apple Filing Protocol will no longer be the default protocol for sharing files on Macs. Instead, Apple will switch to Microsoft's SMB2, which is, according to Apple, faster and more secure than AFP, and on top of that, it also increases compatibility with non-Mac machines on a network.
All in all, Mavericks seems like a decent update - nothing revolutionary, but more than enough goodies to be interesting. There's also a number of interface tweaks to remove analog design from the operating system, which is a huge, huge plus for me. I can't wait!