(Author's Note: Most of the following editorial is based on my observations of both major US rating systems. Both are very subjective, as is my opinion of them. All comparisons of them, therefore, are a matter of opinion. Feel free to disagree. :) )
Background
For those readers who are not familiar with them:
PG-13 was added to the MPAA ratings in 1984 in response to "Gremlins" and "Indiana Jones and the Temple of Doom," which were not intense enough to meet R-rating standards as written at the time, but violent enough to spark public protests at their PG-rating.
NC-17 replaced X in 1990. The original X-rating was never "officially" sanctioned by the MPPA; consequently, the adult film industry adopted it (and modified it to XXX), making any so-called "legitimate" film to receive the rating difficult to distribute. Many filmmakers (such as George Romero) with movies too intense for an R-rating would refuse to submit their films, making distribution difficult among major theatre chains almost impossible.
The ESRB rating was established in the late 1990's. Although the idea started with the release of "Mortal Kombat" (with it's over-the-top "fatalities") and "Primal Rage" (and Chaos' "urinality" move specifically) for the consoles, it was officially put in place in response to the Columbine school-shooting, and the insinuation that "Doom" was the major reason that the teens went on their killing spree.
For the best comparison, I'll focus most of my attention on the ratings that impact me the most as a parent: the MPAA's PG-13 and R ratings, and the ESRB's T and M ratings.
Comparing the Rating Criteria
This is one of the bigger flaws in the ESRB: while the MPAA site spells out exactly what will result in each rating, the ESRB is annoyingly vague.
Quoting the MPAA's PG-13 criteria (http://mpaa.org/FlmRat_Ratings.asp): "...The theme of the motion picture by itself will not result in a rating greater than PG-13, although depictions of activities related to a mature theme may result in a restricted rating for the motion picture. Any drug use will initially require at least a PG-13 rating. More than brief nudity will require at least a PG-13 rating, but such nudity in a PG-13 rated motion picture generally will not be sexually oriented. There may be depictions of violence in a PG-13 movie, but generally not both realistic and extreme or persistent violence. A motion picture's single use of one of the harsher sexually-derived words, though only as an expletive, initially requires at least a PG-13 rating." (This is the infamous "f-bomb.") "More than one such expletive requires an R rating, as must even one of those words used in a sexual context. The Rating Board nevertheless may rate such a motion picture PG-13 if, based on a special vote by a two-thirds majority, the Raters feel that most American parents would believe that a PG-13 rating is appropriate because of the context or manner in which the words are used or because the use of those words in the motion picture is inconspicuous."
Quoting the ESRB (http://www.esrb.org/ratings/ratings_guide.jsp): "...content that may be suitable for ages 13 and older. Titles in this category may contain violence, suggestive themes, crude humor, minimal blood, simulated gambling, and/or infrequent use of strong language."
Also of note: While the MPAA implicitly states that depictions of drug use are "suitable" for teens, the ESRB does not. Therefore, a game targeted at teens showing the harmful effects of drug abuse could, in practice, draw an M rating, defeating the purpose of the game.
The Ratings in Practice
Over the years, I've found both ratings systems have extremely different results given the same material to work with.
Violence: I felt that the PG-13 rating given to "Aliens vs. Predator" baffling, given that the movie was based on 2 franchises that, traditionally, received R ratings. Watching the movie, I'm still astounded that the level of violence was on par with "Aliens" (complete with a chest-burster scene) and "Predator," and it still drew a PG-13. By contrast, the PC shooters from the same franchise, with the same level of violence as the movie, received an M rating.
Even despite the ESRB's assertion that "minimal blood" is suitable for a T rating, I have yet to find a game with any bloodshed that has drawn less than an M.
Alcohol and Drug Abuse: The only game I've found with alcohol use that was rated T is "Guild Wars." Other than that: "Bioshock" (with its Eve injections), "Narc" (using drugs to bust criminals, complete with adverse side-effects and withdrawal symptoms)... rated M.
Adult Humor: If "Destroy All Humans!" was a movie, it would have rated R by the MPAA: The main character "probes" humans for DNA to buy upgrades; many of the characters' names are very suggestive (such as Coyote Bongwater); the obscene phone calls made to throw off the authorities (though not on the level with similar calls, such as the one made in "Porky's).
Sexual Content: Having recently purchased "The Witcher," I'm completely baffled by its M rating. The US version of the various sexual encounters (as well as their cards) is on the level of many PG-13-rated films (including big-budget blockbusters such as "Armageddon").
Legal Status
The biggest difference between the MPAA and ESRB is their status. While the MPAA ratings are strictly "voluntary" (you can choose not to submit a movie for rating, but it will not be released by the major theatre chains), as I noted in a previous editorial, New York has made the ESRB rating mandatory on all games offered at retail.
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In my opinion, the ESRB needs a lot more work, and a lot more consistency with other rating systems, before it can be used on its own as a means of monitoring what my children play.
The best method I've found (and the rule in our house) is: I play the T- and M-rated games first. This way, I know what to expect from the game, am fully informed as to the game's content, and can answer any questions that arise from the game. If I feel uncomfortable letting our teen-aged son play it (this happens rarely, but it does happen), it's "off limits" until further notice.
(Author's Note 2: Please read the next paragraph carefully before commenting on it.)
A game like "The Witcher," for example, is a definite "no" until I play all the way through the game, and can explain what the main character is doing in the context of the story and setting. This isn't a hypocritical "no sex allowed!" reaction; it's a measured decision based on my level of comfort with the events of the game.
I would expect other parents to follow my lead. However, I wouldn't expect a non-gamer to suddenly, overnight, gain the kind of experience in games I have from 30+ years of video gaming; therefore, the ESRB, as flawed as it is, continues to be the preferred measure of what's appropriate.