The AO for Adults Only rating means that, according to the ESRB's official definition, the current version of the game now "should only be played by persons 18 years and older"...
Oh my... that sounds... exactly like the "Restricted 18" rating this game has already got in my country.

Oh teh noes. A game in which your character can be rewarded for mass murder, brutal robbery and slaying of innocents, and pimping/murdering prostitutes gets a restriction placed on it? How unthinkable.

Even though the new material's only accessible via third-party interference, it's gotta be examined when assessing the game. If not, lame game companies who want to take advantage of the "OMG CONTROVERSY = FREE PUBLICITY" campaign will just start loading their games with stupid crap that you can't access through conventional means, then 'subtly' advertising the fact.

I don't have a problem with the current game classification systems. In my country, it's goverened by the Films, Videos and Publications Act, which applies to basically any other media as well. Products get a warning label as appropriate, and a legally-binding restriction (eg. 16, 18 or not legally available at all) where appropriate. It works fine.
The only problem is parents who are utterly stupid and buy their children whatever software they want, thinking that "games" are all about fun, without bothering to read the censor's warning or even read the back of the box. "Omg! I bought my 9-year old son Puppy Molester 3 and Sandbox Tentacle Tyrant, and it turns out they've got violence and sexual perversion in them! Oh, the horror! I'd better sue somebody, since that's what stupid people are supposed to do in times like these."

A little responsibility goes a long way.