Metro apps are easier to navigate with a finger than with a mouse.
The interfaces in most Metro apps are wide and are built to scroll horizontally, sometimes through screen after screen. This arrangement works great when you're navigating with a finger on a tablet; but with a mouse, not so much. Your options are to use the scrollwheel on a mouse or the scrollbar at the bottom of the page. I'd be able to left-click an empty area of the page, grab it and pull it to the side horizontally, as you can in many Adobe applications. To me that seems like a closer analogy to the way people navigate pages by touch.
All Metro apps display at full-screen size and can't be moved from one screen to another.
The look is striking and gives the applications lots of breathing space. But sometimes you need to see two programs at the same time, to compare information or to move data from one application to another. You can grab the top of an application and move it so that it sits in a vertical panel on the side of your screen, but that orientation isn't useful for most programs.
Metro apps look pretty, but their information density is often quite low.
The Music app is one example of a Windows 8 app that supplies a low density of information per screen. Albums, artists, and even songs appear as an array of tiles. On my wide-screen monitor, I can see only 24 tiles at a time. To see more albums, I have to scroll horizontally; and if I scroll too fast, I see just generic gray tiles, which persist for a few seconds while the app populates the tiles with album names and art. Once the images are in place, the app has an attractive look, and you can narrow your focus by searching by genre, but it's not an efficient way to scan a library of hundreds of albums.
Other Microsoft apps, such as the people and photo apps, have a similar design, and third-party designers are obviously reading from the same playbook. The Evernote app, for instance, is almost unusable if you need to find an older note from a large collection. It shows just 14 tiles per screen on my system--and the notes are arranged strictly chronologically, with no search function. Surely, Evernote will do better in future updates. But the underlying problem is that the apps' design motif, while great for tablets, doesn't make good use of the capabilities of a real PC.
Windows 8 menus are contextual.
If you click the Settings icon while you're in the Metro start page, you get settings specifically for the start page. You can click a link below for 'PC Settings', but those settings don't include everything you're used to having access to in the Windows Control Panel. To obtain a link to the Control Panel, you must click the Settings icon while you're in the traditional desktop.
Perhaps this is something users will become accustomed to; after all, it is how many mobile apps work? But I think users expect more consistency from their desktop OS.
How much can Microsoft fix?
I want to stress again that, appearances to the contrary, I really like the Metro interface. I think that many of my gripes involve problems that Microsoft could probably fix fairly easily, perhaps by the time the final version of Windows 8 ships. Other issues, however, such as the tablet-oriented design of applications and the fact that the Metro and traditional desktops feel like separate operating systems, are more fundamental and make me wonder whether Microsoft can come up with a solution that makes Windows 8 feel like a cohesive unit.