Originally Posted by MacApfel
Could you emphasise what exactly you find gorgeous about it. From what is announced, it doesn't seem too different from ML.
The most important feature for me is clearly multi-monitor support and the ability to treat AirPlay devices as a monitor in this context. Since the appearance of Mission Control and Full Screen Apps, and the removal of multi-monitor features in QuickTime X, working, or even showing movies, on systems with more than one display was a disaster (actually worse than anything Macs could do even under the classic OS or just any version before 10.7).
In 10.9 they, IMHO of course, finally got it right. Not only from an OS X perspective, but miles ahead of any other OS. Each display has its own Spaces, Dock and Menu Bar, Mission Control can move stuff between all of them, there are System Preferences to fine-tune that behaviour... for somebody, like me, using multiple displays most of the time, this is huge. And it is extremely well done. That alone would justify an (old times) $129 upgrade for me... and it will cost far less, I assume.
For somebody having a mixed network in most places, the use of SMB 2 (iso AFP) as the default file sharing protocol is also quite substantial. Our 10.9 beta machines achieve much more consistent throughput in our test network.
iCloud Keychain is another feature I absolutely adore. Yes, there was Keychain syncing in .Mac and MobileMe before, and yes, if Apple would allow third parties (like 1Password) to hook into Safari on iDevices, they would have solved that problem long ago... Still, this is now a solution that scales well (serving millions of iCloud users, iso of just some die-hards paying $79 p.a.) and is more fine-grained (e.g. with intelligently treating credit card data and security codes separately, suggesting secure passwords etc.).
The next highlight (for me) is that all of the "Advanced Technologies" Apple touted really seem to work (no matter if I truly understand them or not, the latter being more likely). Battery life on my rMBP 15" is up significantly (I would see at least 20% with regular use, this might not be visible in tests or benchmarks, as it mainly benefits from idle phases or low activity, which benchmarks normally do not measure, but it is "there"), memory seems to be freed more rapidly (heavy multi-tasking is much more responsive), gestures, scrolling and zooming are clearly more responsive, pretty much "real time" (felt, not measured) in most cases. It may be hard to describe, but people often say the true art is to make difficult things seem simple. 10.9 does things flawlessly, were you could feel 10.8 was losing quite some sweat in the background.
The "Maps" integration throughout the system is much more useful than I imagined (I thought I wouldn't care for it at all, now I am using it all the time) and since we started to internally publish several of our SOPs and stuff as iBooks files about a year ago, even the desktop iBooks app and its reliable syncing of notes and bookmarks has proven to be quite a killer app (something I would have flat out denied initially).
So, in summary, for me there is a huge difference between 10.9 and 10.8 and almost all of it is enjoyable. From an corporate/admin perspective it is also quite a relief that there seem to be no relevant changes to the device driver architecture. We do use a lot of stuff here that is likely to cause trouble (MFPs, huge plotters, audio and video interfaces, screen and printer calibration devices etc.), and until now (knock on wood) absolutely everything just continued to work.