After upgrading to Xfce 4.10 all items on the Xfce panel were align to the left. This happens because in Xfce 4.10 the "Window Buttons" panel item no longer expands, one has to add a separator and set this separator to expand.
Open a terminal window and run:
Things should be back to normal by now. But if the problem persists, open the panel preferences, check if there's a separator after the "Window Buttons" item and the separator should be set to "expand":