Because I know those of you who care about this shit have already read dozens of other reviews from people who actually get paid for posting their opinions online.
First off, it works. In my opinion, that's the best compliment any new OS can receive. In my week-long experience with it so far, nothing's broken. Unlike Windows, which pretty much requires any OS upgrade to be a "clean" install if you don't want issues, Mavericks just laid over Mountain Lion and came right back up. Which is exactly what I have come to expect from the folks in Cupertino.
There are hundreds of changes, but on the surface it looks pretty much the same as Mountain Lion. There are few subtle visual and functional tweaks here and there, but if you weren't looking for specific things and just sat down in front of two machines running the different OSes side by side, they'd look almost identical.
I find one of the big advertised features—Tabs in Finder—helpful, because the Finder was one of the things I hated most after switching over from Windows four years ago. Copying and moving files is so much easier now, but still not as easy under Windows. (Did I just say that?) Why there still isn't an option to simply display a tree structure at the left of the Finder window still amazes me. (If you absolutely must have a Windows Explorer-like file manager, check out MacIntosh Explorer. The program is a little long in the tooth, isn't optimized for retina displays, and occasionally has issues (it's no longer in active development), but if you want immediate access to all those files and folders that Apple thinks you don't need to see without having to go to the Terminal to manually un-hide things, it might be what you're looking for). I use it only rarely, but when I need to, it does the job with a minimum of fuss.
I haven't used dual monitors in nearly a year, so I haven't had a chance to check that bullet feature out.
Likewise, I use 1Password as my password keeper, so iCloud keychain is superfluous to me.
I'm happy to see the faux leather and stitching finally gone from Calendar and Contacts. Also gone is the yellow ruled paper from the Notes program and the gray linen background from the log-in screen.
Having Apple Maps in iOS is pretty much irrelevant to me, although I do keep the icon on my dock in case I just need to look up a quick address. As for the 3D flyovers and the satellite imagery, the visuals aren't any better than they are on iOS. "Only as good as the data" is still true here. Since they now have Maps on the desktop and Find my i-Whatever available through iCloud, I'm hoping that at some point they integrate Find My Friends into OS X as well, although at this point I'd be just as happy if they simply ditched the stitched leather interface of the program in iOS and brought it in line with the rest of Apple's iOS apps.
I don't use iBooks, so that was removed from the dock within seconds of the post-upgrade reboot.
The only things that don't work for any more are two third-party screensavers (Fliqlo and Colour Clock) I had installed, and that's because they relied on Flash in order to run. And if I don't absolutely need to have that resource hog installed, I won't install it. Not a huge loss; I've found suitable replacements.
I also received the iWork and iLife updates. Unlike a lot of the fanboys, I'm not grousing about the changes they made to the iWork apps—probably because even after four years I have yet to fully embrace them as my main productivity tools. Believe me, I've tried on more than one occasion to do it, but with twenty years experience under my belt, whenever I need to sit down and do real work, I fire up Word and Excel. As much as I have come to despise all things Microsoft, sometimes it's easier to just go with what you know.
So that's my two cents. As I said, I'm sure there are hundreds of improvements behind the scenes, but nothing that has really leapt out at me or intruded upon my consciousness. I guess that's a good thing, no?
Maybe one of these days I'll switch to Mac. Maybe. 😉