* When a maximized window is behind an active non-maximized window, the panel serves as its title bar. But it actually spells out the name of the active window. Very confusing.
* Additionally, in the same situation, all of they standard mouse gestures on the panel (e.g: double-click to unmaximize) do not work. Maximized windows don't get the right-click-title gesture to work at all.
* No window list at a glance (useful for remembering your context/state with the UI)
* Hides important buttons such as close window, as well as the menus, where non-power-users will not know to look.
* Panel unhide takes a relatively long time -- slowing down almost every interaction with the UI.
* Workspace switcher does not support window drag, unlike the gnome panel workspace applets.
* Workspace switcher requires multiple selections to switch to a window.
* Alt-tab switches applications, not windows, which is more confusing, as it associates unrelated windows together, as if the mere fact they're implemented by a certain application necessarily means they relate. This also requires more keystrokes to reach the window you want -- when you don't have many of them in a workspace, which is a typical situation for organized users that actually use workspaces.
I could go on and on. Unity is an immature, alpha-quality piece of work that seems to have been thrown out because of a deadline.