, AndyC wrote

*snip*

No, but that's probably because the mouse pointer is naturally very close to the thumbnail by the time it appears (because it had to hover over the button to trigger it). As a result you're typically making a shorter, slower movement and are thus more likely to accurately hit the correct target. By contrast the IE tabs or the W8 app list potentially require a much longer range movement and thus accurately targeting quickly becomes more difficult. You may have heard of this before, it's called Fitts Law.

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.

Also thumbnails have much more vertical room in which you can't accidentally click a close button, as opposed to tabs.

PS: I noticed that Windows 8 App Store App Internet Explorer 10 has permanently displayed close buttons on its thumbnail shaped tabs.