Multi-tasking in Metro can be a frustrating experience. In the Metro UI Microsoft takes a similar approach to Apple by offering full-screen, immersive experiences. To save battery life, apps not currently in use are frozen. Most of the time this is fine; switching back and forth is seamless and fast. Where it gets weird is when you want both to run at once, even if you're not looking at it.
Unless a program is specifically written to do so, moving away from an app pauses it. So, for example, if you wanted to read headlines in the news app while listening to a music video on YouTube (a situtaion I actually ran in to not long ago), you have no choice but to use the snap feature Microsoft has built in for running two things at once. It's not possible to simply start the video on YouTube and switch entirely to another Metro App.
