Let it be known that I'm not declaring war on the ESRB. In fact, compared to the MPAA (the film rating board), the ESRB is a fairly reasonable organization that has, for the most part, helped gamers. Its "federally suggested" formation in the early 1990s helped relieve pressure from the US Congress, who were growing increasingly antsy about the higher level of violent detail in the 16-bit era. It's a self-regulating organization whose ratings procedure is fairly open, especially when compared with the MPAA. And unlike the FCC, the ESRB is run and maintained by the video game industry, thus leading one to believe that it has good intentions for the business as a whole.
Yet one should be aware of the great power the ESRB wields over free speech. Although there are no legal ramifications to producing a game that the ESRB rates as "Adults Only," such games are essentially dead in the water. No major retailer which carries video games will stock them, and it's unlikely that any independent retailer would bother to go through the trouble to acquire such games. Furthermore, the three major console publishers would never allow an AO game to be published for their system (assuming that they knew about it beforehand). Indeed, only a handful of games have ever received the rating, and most of them were made for the PC. A game given the dreaded rating will often be forced to alter content so as to acquire a "Mature" rating instead, thus granting the ESRB de facto censorship powers.
More recently, the ESRB caused shockwaves when they bullied Gametrailers.com into removing a trailer for Fallout 3 from its site, claiming that it violated the organization's "advertising guidelines". I'm no lawyer, but I'd assume that a website should be free to run any content that it pleases, so long as it's not breaking any laws. The ESRB knows this, so it decided to go through Fallout 3's publisher and have them "request" that the trailer be taken down. I've seen the trailer in question, and it's no worse than any R-rated zombie flick I've ever seen. Something about this just reeks, and the ESRB isn't commenting on the situation. I think we owe it to ourselves to keep an eye out for any more ESRB-sanctioned censorship in the future.