#4 - Switching between screens is a nightmare
Switching between Metro apps is a kludgy mess on an epic scale. You don't have the traditional Alt-Tab combo, but instead you take your pointer (or finder, and place it on the left-hand side of the screen and either drag across or click on the icon that pops up.
Sounds easy ... but wait!
Problem is, once you have more than a few apps running, there's no quick and easy way to cycle between them. You move the mouse to the left, an icon pops up, and if it's not the right one, you have to click on it and try again.
This is so amazingly kludgy that I can only hope that it's some kind of stop-gap. On a tablet, this is ideal, but on a desktop or notebook, it has to be one of the worst design decisions I've seen in ages.
Microsoft - FIX IT!
#5 - Tiles will eventually lead to bloat
Remember when Microsoft introduced the System Tray. It was a place for important stuff to live so users could have easy access to it and see the status of certain applications. Pretty soon, this prime-time real estate (prime-time because it attracted eyeballs) was filled with all sorts of crap.
Looking at Windows 8, the Metro UI desktop will become the next System Tray. Every app is going to want a piece of this real estate. And more apps running will eventually lead to bloat. While Microsoft will surely make sure that the apps it ships with Windows 8 will be lightweight and play nicely, imagine when you have you have iTunes and Steam and so on running, bombarding you with ads and crap news and information. It's a dead certainty that OEMs will be pre-installing apps in exchange for cash, bringing a new era of crapware.
Will Microsoft prevent OEMs from filling the Metro screen with crap? I hope so!
#6 - There needs to be an option to turn tiles into icons
The smallest tile is too big ... once you have dozens of apps installed, that Metro screen is going to get mighty cluttered.
Also, there needs to be a logical way to categorizing apps beyond bundling them into groups.
How tiles are handled needs to be streamlined to make it easier to navigate.
The more I use Windows 8, the more it feels like Windows 7 with a Metro bolted onto the side. And Metro feels like a hybrid between the Windows Phone OS and the Xbox 360 interface. It feels weird and unfinished.
If Windows had a full Metro UI, completely replacing classic UI then I could see it being really useful on tablets, but right now it's little more than a veneer, and by making Metro the default on desktop systems, Microsoft is acting like a child waving a crayon scribble in the faces of any adult that will pay attention. There's no need for Metro to be on the desktop beyond the ability to run apps, but it's there.