To me, there are two major negative values promoted by television.
First, there is the idea that violence, promiscuity, drug use and other negative behaviors are cool and acceptable. TV shows tend to show a great deal of these kinds of behaviors, often without showing any consequences of those behaviors. This is, for example, why TV is often said to cause kids to be more violent than they otherwise would be.
Second, TV promotes materialism. As people watch TV, they see people (in shows and especially in ads) enjoying lifestyles that are based on material possessions and which seem very attractive. Seeing this makes people want those things -- it helps make them define their lives in terms of what material possessions they have.