Show a nice "x" on the thumbnails that I can click to close the app (like Windows Store IE's tabs) without having to right-click to bring up a context menu.
Provide jump lists like the desktop's task bar.
But that's just a lame attempt to reinvent the Taskbar and bring back a whole bunch of legacy behaviours that Metro apps are intended to get away from. Having to manually manage application lifecycles is just stupid, if that behaviour hadn't been thrust upon us out of necessity for the last 40-odd years nobody would think it actually makes sense. It's like having to manually remember to save documents so that you don't lose your work, once upon a time there was little choice but to write applications like that, simply because you couldn't guarantee that a storage medium was even available at any given time and even if it was, it was too slow and limited to be something you could take advantage of. Those days are long behind us and yet we're still forcing end users through the same old ridiculous rituals - it's the cargo cult mentality personified.
Imagine for a moment if your TV worked like that. You had to manually "close" channels when you weren't watching them and you could only flick between the ones you currently had "open". To switch to a channel you didn't have open, you had to go into a separate menu and start it up. Of course you'd also have to be careful not to have too many open at once, because even though you're only watching one having too many open would cause things to get slow and jittery. And you'd also have to be careful not to close ones your PVR was set to record, otherwise it would just fail. It's ridiculous to even think that would be better and yet here we are in the 21st century with people actually arguing the PC is better if the OS doesn't do it's job and manage resources for you.