My machine at work runs Ubuntu (instead of Kubuntu), as that was what someone installed for me before I got there, so when I updated to Maverick I got a chance to work with Unity.  I don't hate it, but I find it annoying in a number of small ways:

• I can't see all my open windows and directly target switching with the mouse. The "task bar" in Kubuntu lets me see everything that is open, and if I am on the trackpad/mouse I can just mouse to the taskbar-item I want (I disallow collapsing by application, which is easy to discover in KDE, but I haven't found the equivalent for Unity yet) and have it open (if you are counting, that's two "locates" and two "moves").  In Unity, I have to click the application on the left, then find the particular instance in the shown windows, then mouse all the way back to it, then mouse back to where it shows up on the desktop (three locates, three moves).
• In the default setup this is even worse, as the dock is auto-hiding, so the mouse operation to switch windows is "mouse to edge, wait, locate icon, move to icon, locate window preview, move to window preview, locate window, move to window". (Fixing that required running compiz configuration... I would *not* have known to do that if I hadn't googled it).
• Starting a new instance of an application is non-discoverable.  Turns out you middle-click to create a new instance.  I had to google that and then happen to know that the scroll-wheel is a middle-button.  On my laptop I haven't a clue what the middle button mapping is (luckily it's running KDE).
• Menus are hidden until the the window has focus.  On big-screen setups, where you often have one application on the left and one on the right monitor, this is an annoyance.  I can't target the "view" menu in Eric until I bring Eric into focus, adding an extra click, and a pause before I can do the targeting for the point on which I want to click (with the menus always visible you can start moving the mouse in the correct direction, find the target as you are moving and refine the mouse movement to hit exactly so that it flows as a single mouse movement).
• Navigating the software menu (Ubuntu menu) is slow enough that I never use the navigation.  It's faster to just take my hand off the mouse and type in a name.  In Kubuntu and previous Ubuntu I often use the menus to trigger less-commonly-used programs, in Unity I find myself adding everything I use at all to the dock so I won't have to break out to searching when I want it.

They're all just annoyances, but together they are making me want to replace the desktop with KDE.  While out of the box it shares a few of these papercuts, it's trivial to fix them on KDE as I can just right-click on the offending item and tell it not to be silly.