I find this topic fascinating because I've heard so much about TV both positive and negative for many years. We have never had TV at my family home and so in a way it's never been an issue for me- I have no strong feelings about it one way or another, except that obviously staying plugged into the TV is a bad thing if you're watching cruddy shows etc. When I had my own place in university with mates we had a TV and I can't say I ever watched anything enjoyable apart from reruns of movies, which aren't really TV at all.
I guess there are two cases to be made against TV. One is the fact that people waste a heck of a lot of time watching rubbish TV, a lot of which is insidious (I'm thinking of UK TV here, I'm not a prude, but a lot of morning TV here I would turn off because its so immoral) and as Catholics we should safeguard our homes against occasions of sin. I honestly think TV can be an occasion of sin (and often is) because not only are there many blatently immoral images and adverts on there, but because even the airtime which is not immediately offensive is filled with an atheistic and purely materialistic sentiment. It's like a constant barrage of drivel
The other reason is that TV drains kids (and adults!) of imagination. Books, even radio are "hot" media because they require one to engage actively with the mind. TV and movies are "cold" media because all the work is done for you. Nothing wrong with a good movie, but exercising choice power in watching a DVD or going to the cinema is very different from sitting in front of the TV and watching soaps. I know that's almost a philosophical point and doesn't smack of hardcore "resisting evil" but I think it's an important one too.