Unity is a great UI on a netbook screen. Things like the global menu and maximize behaviour make best use of the limited space. BUT not everyone has a netbook and Unity sucks rocks on anything larger.
Global menus incur an annoying amount of mouse travel on large screens and you can't even see an app's menu unless the app is the active window and you mouse up to the top. It's just bad design.
Other annoyances would include:
Hover scrollbars. They are awful. You have to look back as far as Open Look WM to find a scroll bar user hostile as this monstrosity.
Unity is a space hog. What looks good on a small netbook screen looks ridiculous scaled up over 4x+ the area. For example click on the Unity button and the entire screen is eaten up to display 8 lousy icons.
Ubuntu store is EVERYWHERE. Click on the Ubuntu icon with the intent of running apps on your own machine and observe as half of the space is occupied suggesting apps on the store. Sorry Ubuntu but you're just being annoying. Put a store tab in the panel but leave the rest alone.
So sorry Unity you suck. The sad part is that some better defaults and some settings available from a simple configuration dialog could do wonders to make it somewhat pleasant to use. How hard is it to have checkboxes to disable global menus and those godawful scrollbars?
I still use Ubuntu but these days it's with GNOME Shell 3.2. Despite it's bad rap, GNOME shell is a remarkably usable interface. It too has some deficiencies and needs tweaking but generally speaking it's a far better thought out thing than Unity. By miles.