So far, my biggest gripe with KitKat is that there is literally zero customization available for the launcher, and if you switch to something like Nova (my previous choice), you lose the 1) easy access to a Google Now screen, and 2) "Ok Google" voice search. This wouldn't ordinarily bother me much but there are two big UI/UX gripes I have with the KitKat launcher:
1) To quote a Reddit meme, the icons are just... too damn big! Yes, the screen is lovely and the new icons are perfectly good looking, but they're huge! Even on a 5" 1080P screen, you are only allowed a 4x4 grid. I used a 6x6 grid on my Galaxy Nexus and think that size is far more appropriate, especially if you want to take advantage of multiple widgets on your homescreens, or a mix of widgets and shortcuts on the same homescreen.
2) Related to the first complaint, you can't create app folders in the app drawer. This makes it impossible to "hide" things like LightFlow, News & Weather, Keep Screen On, the default Email client, or other utilities and random apps you don't ever want to see. At least you get five rows of four, but still, the icons are infuriatingly large.
I haven't played with the camera enough to comment, and the battery is light years better than my Galaxy Nexus so I have no complaints there (yet). I do carry a Powerstick (http://www.powerstick.com) in my briefcase when traveling, though.
The fact that you can now do both video and voice, or just voice, Hangouts while on cellular networks is enough for me to overlook quite a few issues.
One last tidbit: the new dialer & phone app is much, much better than the old stock one (imho, this is something Samsung really did a good job of with Touchwiz), but it is not intuitive and there are some wonky things about it, like the fact that the standard dialing screen only shows 3 of your starred contacts and there is no apparent way to scroll/view the rest of your favorites.
I'm expecting a 4.4.1 sometime not too long from now because there are enough things lacking polish to warrant a few extra weeks in the oven.
As for the hardware, not a huge fan of the sharp edges, but it does feel good in the hand and the small side bezels make it feel smaller than a 5" phone should.