So theres a new bill out saying that the ESRB must "play through" the entire game before rating it. this won't help for several reasons:
1)some games are extremely long(the elder scrolls games for example)
2)some games simply don't have an ending (most MMO's like World of Warcraft
3)user created mods aren't even mentioned, which generates the majority of "bad" content in games
heres a web comic that discusses the issue:
another reason why ratings in general don't work for video games is that they are so interactive that for the most part players choose the level of violence they wish to incorporate into the story. In Grand Theft Auto for example you can choose weather you want to get from point A to point B by obeying all the traffic laws, or run over anything in your path and take shortcuts through crowded sidewalks.
Rockstar now offers the original Grand Theft Auto for download(free) vary little has changed in terms of how the player determines the level of violence, if you doubt my explanation, try it for your self:
http://www.rockstargames.com/classics/