First, I don't think television in and of itself is bad. I do find there to be a lot of things I don't agree with and/or don't want my children watching. I find that television can be used for good, but also bad. I do allow Brayden to watch some television each day. He watches about an hour. I have found if he has more than that, he gets to be less obedient, and more interested in being disobedient. It isn't the shows he is watching. He only watches PBS Kids or movies. So it isn't the shows, but the actual television that causes the problem.
Kaitlyn doesn't watch television (though she has seen it). Some day I will let her. Brayden didn't start having it as a regular part of his day until he was 19-20 months old. He started watching it on occasion around 15-16 months old.
Television is physiologically addictive. We all know that. It is good to set limits so that we do other things. Watching some TV each day is fine (at least in my eyes). You can find good things to watch that educate and entertain in a good way. But you don't want it to become the main part of the day. In the United States, it is reported that the average child watches approximately four hours of television daily--that is just television. Watching movies and playing video games are a separate number (American Academy of Pediatrics, “Television and the Family,” 1, www.aap.org/family/tv1.html).
Does that sound like a lot of time spent sitting in front of a TV to you? It does me! I doubt that parents prefer their children to just sit in front of the TV for the majority of their waking hours, but it happens. It happens because it is addictive. Setting limits realistic for your family is a great idea. There are so many other things for children to do. Coloring, playing with toys, listening to music, playing with play-doh, playing outside, reading, homework, chores, friends, walks, etc. We all know. You can't possibly fit it all into the day even without TV in the mix. Television can teach, but it is absorbent learning rather than interactive learning. There are theories that learning problems in the classroom stem from children expecting to be entertained into their knowledge, rather than having to work for it. All of these activities and more can teach your child more concretely.
Some nights my son will ask to watch a show, and I will suggest something else instead. He always says, "YEAH!" He doesn't prefer the television to other activities, but it is the easiest and most entertaining thing for him to turn to. It is a lot harder in the winter (at least where I live--we have real winter). Outside time is limited or non-existent. But a little thought (and honestly a lot of effort on your part) can get you through!
Even seemingly harmless shows can teach children things you don't necessarily want them learning. You do not want the majority of the lessons learned by your child to come from the television. A lot of violence can be seen on television in our modern times. We all know children pick up and learn from television, for better or worse.
So my opinions on television (and other media) are to limit the amount of time and also to always know what is being watched and/or played. You are the parent; you set the rules.