The most meaningful dimension is the one that's going to be the most limiting one. X1 is very wide, especially if you take into account the PSU.
I don't like putting actively cooled things in place where they just suck up dust, and anything large and boxy I generally like to arrange in an orderly way.
There is typically free space behind and above all consoles when they're in place. In practice, that's the way it works in pretty much every tv stand and cabinet. Once the console is in there, the space above and behind is unusable to anything else. The space to the side is where you fit more things in. The usable space an X1 voids off is unprecedented.
I can see my PSU. I can see most folks' PSUs come to think of it. Perhaps things are different in the US vs Europe, but lightweight tv stands that you can see into and see through are common here, and as residential spaces are much smaller here than in the US some people have even taken to putting tvs directly onto walls and avoided having "entertainment centres" entirely.
It's also common for kids to want consoles in their bedrooms here, and they wouldn't typically have entertainment centres in their bedrooms.
I'm certainly not alone in this. Phil "I believe in" Spencer has said openly that they received lots of feedback that customers wanted the power brick gone. So much, in fact, that they engineered an internal power supply into their new system for the first time in a decade, bringing them into line with the far more successful Sony. So at the end of the day this is a thing that even MS themselves agree with.
Edit: I would certainly be interested to see if X1S boosts sales more in Europe than in the US. In the "pokey roomed" UK its done very well so far, but it remains to be seen if it'll have legs.
Legs would make it even harder to fit into a entertainment centre, although I guess it could aid portability.
