The blurb here is wrong. The Critical Detail post quotes Microsoft as anything rated above M or PEGI-16.
”Your app must not contain adult content, and metadata must be appropriate for everyone. Apps with a rating over PEGI 16, ESRB MATURE, or that contain content that would warrant such a rating, are not allowed."

So that means games rated ESRB AO and PEGI-18.

Regardless, this has no impact on the PC platform. If you were intending on buying an RT tablet and wanted Larry Flint's Hot Chowder Simulator v69, then I guess you're out of luck. How many PC users are going to exclusively use the Modern fka Metro interface and/or buy apps for them? Very little - and the only apps that PC users of Modern UI are going to purchase are the same ones they want a homogenious experience with across their Win 8 tablets and phones.

All Microsoft is doing is indemnifying themselves from lawsuits if craptastic parents can't monitor their children and some pr0ns are downloaded through their store. That and any other behavior the same craptastic parents think hurts their preciouses. All this chicken little stuff is silly and a bit retarded. The only people complaining are folks too blinded by their inherent dislike of Microsoft to look at this objectively, or companies whose business model may be in jeopardy.