So with the old start menu, users were looking at the start menu when they opened it. That makes perfect sense -- why else would you open the start menu if you didn't want to look at it? However, it also took up a smaller section of the screen, so you could more readily find what you are looking at. With the Metro screen, I have to play Where's Waldo whenever I want to find an app (I have the same problem with Gnome 3 shell, trying to find an app icon in that massive wasteland).
But that's just me -- I also didn't like the sliding start menu design either (Windows 7, KDE 4), where submenus open in the same space as the folder icons -- I'd rather, when clicking on a submenu, have it open into new screen real estate like any of the traditional menus.