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.
One thing that I would add to the first answer is that you can argue that watching television makes kids less likely to think.
The idea here is that watching TV is a completely passive experience. A kid who is watching TV does not need to really engage her brain.
A kid who is reading has to imagine what the things in the book look like. A kid who is playing with toys has to do stuff with the toys -- they are both involved in what they are doing.
But a kid watching TV can just sit there without having to think.
There might be a couple of points that need to be made regarding the impact of children watching too much television. The statistics indicate that children who watch more than four hours of television a day are more likely to be overweight or obese. Another condition that can result from watching too much television is a distorted view of the world. Preponderant images from television can actually influence the child seeing the world in that manner. Seeing that television producers deliberately depict images of the world in accordance to a show's marketability, children who watch too much television are receiving input about the world that has been distorted by commercialism and economic needs. There can be some distinct advantages to watching television, in moderation, though. Public television can be an excellent forum for children to understand more about nature and the natural world, as well as understand how words are assembled as well as understand the recognition of letters and sounds associated with them.