The distinction between "casual" and "real" gaming is entirely facetious, and can actually do more harm to the credibility of your statements than anything. Games are games and come in all shapes and sizes.

I also have to disagree with your dismissal of horsepower as the root of the issue, because I can currently hook up any keyboard, mouse, or even game controller I want to my phone or tablet. Additionally, some portable devices are capable of hooking up to a larger screen via a microHDMI port. Their only limitation, then, would be the internal hardware components that ultimately add up to their "horsepower", with the added advantage of portability.

It's not facetious; there is a huge difference between people who primarily play mobile games and people who build machines for serious gaming. The represent completely different markets. If you were actually trying to sell games with your mentality you would fail miserably. You have to know your market.

Also, if I have a tablet with as much horsepower as my desktop then it would necessarily be more expensive than said desktop. So, you're proposing that I buy a tablet which is more expensive than my desktop and hook it up to all the peripherals I need so that I can... wait, what is the advantage here exactly?

You say that I gain portability; right, as long as I bring along my keyboard, mouse, and a monitor. Now that's what I call portability! Certain games just don't work without a large screen and more advanced input devices. Again, completely different markets.

As an aside, since you obviously took exception to my terminology, note that I put quotes around "casual" and "real" for a reason. I don't mean to look down upon the casuals, but to believe that they are basically one in the same is just ignorant. Those in either segment have very different tastes and want very different things from the games they play. I want games with depth that I can sink endless hours into. My girlfriend wants an occasional distraction. You're never going to sell us on the same marketing lines. We want different things.

Whether or not the distinction between "real" and "casual" gaming is real, there is undeniably a distinction between the sort of games that Microsoft currently seems to be trying to sell through their App Store, and the sort of games that Steam sells.

Surprising to very few, this distinction just so happens to align pretty damn close with the alleged "casual"/"real" divide.

(Also, you've been able to dock laptops for years, that isn't some sort of neat "Surface-like-device" trick, yet all the "real" gamers that I know still own dedicated desktop machines. Their screens, keyboards, mice, and controllers are all bulky. Their setup is already the antithesis of portable, going through all that trouble just to use a docked underpowered device just doesn't make sense.)