According to Google, Viacom's rehashed $1 billion lawsuit against YouTube is a direct threat to the internet as we know it. If the courts ruled against Google in this case, it really would affect the way people communicate. Google's response to the re-filed lawsuit says it best:
Viacom’s complaint threatens the way hundreds of millions of people legitimately exchange information, news, entertainment, political and artistic expression.
The problem is that media companies view the internet as a way to broadcast content, while Google, and the rest of the world, sees it as a form of communication. Copyright laws that usually protect broadcast media are somewhat diluted with the existence of the DMCA which puts the copyright problem in the hands of the communicator, not the medium. In that case, since YouTube is only hosting user-submitted content, they are not liable for it -- besides the fact they must remove copyrighted material when asked to.
As Google states, they have gone far beyond their legal obligations with YouTube by investing heavily in the creation of a content filtering system that theoretically should help copyright owners protect their material.
It will definitely be interesting to see how this case unfolds -- what do you think will happen?