The ESRB, like all ratings systems, are simply outdated, unworkable and obsolete.
Such systems hark back to the days when cinema was new- when media was scarce, needed expensive equipment to be shown at all and international travel was a rare luxury for the super-rich. The idea that media could be controlled in such a fashion was at least feasable.
The second somebody invented VHS, however, it was all over.
Nowadays we have hundreds of television channels, plus video-on-demand, we're onto our third major physical home video format, we have both cheap video production equipment plus the free-for-all that is the internet and if push comes to shove in most of the western world you can get to a foreign country on pocket change. Suddenly, the idea that ratings boards have any real control over anything is laughable- get overly-restricitve on a title? It's off to peer-to-peer we go for the uncut version.
Unworkability comes into the equation when you think about simply how much media that is to cover, as has been pointed out upthread- there's no way in hell the world's ratings boards can get enough manpower in to do it.
And that assumes that people are going to pay to have their stuff rated- independent developers simply can't afford to pay for the ratings, and amateur or hobbyist developers aren't going to- if I want to work in the games industry, I'm not throwing money away on a rating for the little work sample I threw together for my online portfolio. Even if I am, I'm going to go to my local ratings board (in my case, PEGI) and not every single one in every single country.
Finally, there's far better information on the content of games out there on the internet than the ESRB can ever hope to offer- it's easy to sneer at those Christian websites that rate media and immediately say nobody should watch Harry Potter because it promotes witchcraft, but that site will be ten times more relevant to a number of people than the ESRB will ever be, and that's before you get to the idea of pulling up footage from a games site that's covered the title (or even putting a bit of effort in and sourcing a demo) and making a decision for yourself.
The best any of these boards can hope for in the future is to step back and take a more advisory role, and make people take responsibility for their own actions.