Australia is currently considering whether to introduce an R18+ classification for computer games, but I wonder whether the system itself is really set up for games. The trouble is that games can have widely differing content, may allow you to create your own content (innocent or obscene), may be modified after market by patches, and may contain (as the "hot coffee" incident with Grand Theft Auto: San Andreas showed) content that is not normally accessible, but can become so.
I think the classification system needs to inform us more prominently of what the content is, rather than to whom it is deemed appropriate. It would result in a more complicated scheme, rating the level of language, violence, sex and horror individually, not to mention drug use, supernatural themes and antisocial behaviour or whatever else they rate. It would take longer to absorb and would make ratings decisions more complicated, but they are complicated already. It's just that we allow the dumbed-down rating to guide us faster to a less-informed decision.
Mokalus of Borg
PS - A "G" rating is more about what is not included.
PPS - So an "R18+" rating is always going to be more complicated than that.