Nuances? Not so much.
Actually, there's a pretty big difference between a voluntary rating system and a mandatory one. For on thing, if it was a law, it would most likely apply to every stupid little iPhone game, and maybe even free games on the internet. Getting these rated would be prohibitively expensive. Most likely the law would end up being ignored in those cases anyway, but any law that's unenforceable shouldn't be made in the first place.
Currently, nobody is obliged to get their game rated if they don't want to. Of course most retail stores won't sell it if they don't. (Which is kind of ironic given the popularity of unrated cuts of movies around here.) But there is likewise no law that retail stores have to obey the ratings either, most of them do anyway for the sake of their family-friendly image.
Another difference is that when a system is voluntary they can't do like Australia and "refuse classification" to certain media, thereby banning it without actually having to use the B word.
Finally, this law in particular had some even more problematic aspects. For one thing it piggy-backed legal power onto the ratings from the existing private rating organization. It shouldn't be to hard to figure out why that's not a great idea. Also, it required a giant "18+" warning on games considered "patently offensive to children" without even attempting to explain what that means. This would almost certainly have a severe chilling effect.