Hear me out. Did you see the great be honkin' vent on the top-right side of the XBone (when horizontal)?
Very, very, VERY few devices have a major vent on top. Even the PS4, with similar internals, has a completely solid top. I've been planning to build a gaming HTPC the size of the ps4, and I have not found one case that makes sacrifices to size or cooling capability in order to avoid having a large vent on top.
And I think I now why: Obstruction.
I'm planning on building a custom case that is about 7"x11"x2", and just looking at the mockup in sketchup It's obvious that a single sheet of paper can completely shut off cooling.
Based on the pictures of the XBone, there's two minor vents (back, right side), a major one on the top, and a major one on the left side. If the one on top is obstructed, then cooling is significantly reduced.
My cousin usually leaves a game or to piled on top of his PS2
(stuff falls off of the PS3), and I realized that many people (including me) when watching a show will put the case for a DVD on top of the DVD player. So I believe scenarios where something is put on top of the XBone are actually fairly common.
Now here's my guess: There may
exist a scenario where people place game cases or other obstructions on top of the XBone, which obstructs airflow enough to prevent proper cooling. I can't say how likely this is, only that I think it may be possible. It won't be a common as the RROD which just needed a little dust and a warm room, but I think this might affect a small number of people.
And for the number of people who I think this will affect, I don't think the number will be that low. I believe more than half of all users will keep the XBone horizontal, but I don't have a specific number. After that you're relying on people to be tidy enough to not put stuff on top of the XBone, have a general idea as to the purpose of the vent, and/or listen when people tell them not to put stuff on the XBone. I don't think those are great odds.