The Windows 8 App list requires you to put the mouse in the top left corner and then move it vertically along the left edge of the screen to activate. It would be nowhere near the close button of the thumbnails.
With a large display there is quite a bit of scope for mouse travel sideways if you're aiming near the bottom of the list. And it's much worse with touch because aiming is a lot less precise, particularly if you're stretching your finger to reach one of the more extreme ends of the list. Ironically the IE tabs suffer less here because you've pretty much got to take your hand away from it's natural position in order to click them.
PS: I noticed that Windows 8 App Store App Internet Explorer 10 has permanently displayed close buttons on its thumbnail shaped tabs.
Yeah, those I the ones I've already got annoyed by hitting occasionally when use the trackpad on my laptop, because in that case you right-click somewhere and then make a (potentially) large mouse movement to hit the tab. It's probably not quite as bad if you've got a mouse plugged in, but trackpads have never been nearly as easy to be precise with as a mouse (despite Windows treating them identically).