A ton of things...
Addresses in Gmail cannot be tapped and open in Maps anymore. I have to manually copy and paste, and then open Maps and paste the address myself.
Maps will not stop asking me if I want to improve the accuracy of my location. With my previous phone, I told it to stop asking me and I never got that prompt again.
On my previous phone, Browser would open with my homepage every time I opened it. Now, it always opens on the last page I was on, even if I closed the browser or restarted my phone since that browsing session.
I can't find a way to get rid of Quick Settings notification bar. I want the clear button at the top of my phone.
For some reason, my phone constantly notifies that a system update has been downloaded. I try to install it to make it go away and it never does, so I am just stuck with this notification crowding my notification area. I looked for a way to turn off system updates and I can't. It will not stop doing this.
The Alarm does not display how many hours/minutes away it is from going off. It only briefly flashes that information when I first set the alarm. In the previous version of Android I had, it would display this information all the time.
I hate the new version of Google Hangouts. On my previous phone, I was able to run the old Google Talk apk, but it doesn't work in this version of Android.
This is just off the top of my head. I'm sure there are more annoying things if I think about it some more. Kitkat is driving me NUTS. This phone is great -- the battery life is incredible. But this version of Android is a huge step backward.