The metaphor they're going for in Metro is the same as a tablet/phone - you're not supposed to care what's open because it's going to be suspended when it's not active. There are a few exceptions to this (e.g. media players), but those have to make additional API calls to be allowed to do anything while in the background.
I understand that. But part of task management is being able to easily move from app to app, and cycling through a bunch of running apps through swipes or alt-tab being the default way of multitasking is terribly inefficient. There is nothing like the taskbar or expose to simply select what running application you want to quickly move to.
Try this with Windows 8.
Open 3 Metro style apps. Something that won't close automatcially after a few minutes. Explorer, and active remote desktop and control panel are good. Now open 3 traditional Windows apps.
Using the touch "swipe from the left" gesture to move between windows, how many tasks are you moving through? Answer: 4 the 3 metro apps and the one Windows Desktop. The Metro UI sees these 6 Apps as only 4.
Now, use Alt-Tab. It more properly cycles through all 6 apps individual. It doesn't matter if it's a Metro-style app or a traditional windows app. It will cycle through each individual app.
If you are running a lot of apps at once, which is something I do all the time, Alt-tab is terribly inefficient to move between apps. I know I almost never use Alt-Tab or Win-Tab to move between windows today.
If you are using a combination of Traditional and Metro syle apps, and touch only, you will be in situations where you have to swipe between apps to get to the "windows desktop" and them seperately pick apps from the taskbar to move them into focus. This is extremely inconsistant and bad design.
The Metro UI is extremely suited to touch, tablet and other small form factor devices, and for users that merely do a handful of things with their machines. A game , e-mail, a bit of surfing. This type of user or these type of devices will never see a problem since there isn't much to manage. On the other hand, a more sophisticated user on a more capable machine will be hamstrung.
Another problem I have with this new metaphor is that Metro does away with the "desktop" concept. Or at least it's trying hard to do away with it. Many pundits are speculating that MS won't even include the traditional windows desktop on ARM devices, as they will not be able to run x86/x64 apps and only need to run Metro-Apps.
What makes the desktop metaphor so great is that it give the user a natural workspace to manage current tasks, both in the form of running applications and as a place to work and store currently relevant documents and other files. The Metro UI doesn't provide a logical and natural way for me to manage my physical workflow. It is simply something that lets me run and swap between apps. I find the traditional desktop metaphor an extremely valuable too for managing my current projects. It's a virtual extension to my physical desktop.
Another problem with the who Metro metaphor is that it wants us to live in full screen apps. At best, you will be able to partially split screen between two metro apps. This is fine data consumption, light work, or small screens where you need to run an app in that mode for it to be useful, but on a desktop PC, especially one with mutliple monitors, the Metro metaphor is extremely limiting and hinders productivity.
The way Win8 looks today, I will recommend it to friends and family that are extremely casual computer users, or if they want to have a tablet type device. But, to my friends that use desktops and use it to get actual work done, I won't be able to recommend it to them. A number of changes need to happen before I am willing to do that.
-The Swipe gesture needs to cycle between all running apps, not just metro apps.
-There needs to be a more efficient way to swap between apps, for when you have many running. An example would be a gesture, hotspot, or keystroke that would bring up an expose like window allowing me to more quickly and effeciently move between apps.
-There needs to be an easy method for "power users" to treat the metro UI as optional with a way to run all windows applications in either full screen or windowed mode.
There is still plenty of time for all of this to happen, and it may well be the eventual plan. This is, after all, just a developer preview and the first time the Windows Metro UI has been put in the public's hands. We'll see how the development moves ahead.
All this said, I will be running Win8 on my Asus EP 121 tablet and my HP TM-2. It will be especially welcome on the Asus machine. But on my desktop, this still remains to be seen. The way Win8 appears TODAY, I see heavy adoption for tablets and some low end PCs, but general computer usage sticking to Win7 for some time to come..