Too much television can certainly be harmful to a child's intellectual development.
Before a child enters school it recommended that they watch no more than 1-2 hours of television a day. Watching too much television takes away from parental interaction, exploring, playing, and learning how to socialize.
When children reach school age too much television interferes with homework, reading, and spending time with the family. They are also getting less physical activity which can lead to obesity.
There are positive aspects about television such as nature and wildlife programs and current events.
One of the biggest problems with television is violence. If children watch too much television it can teach aggressive behavior in addition to things such as smoking, drinking, etc.
The important thing to remember is that television needs to be monitored both for time spent watching it and content.