Watching TV makes you inactive. You just sit there burning as few calories as possible, which could lead to weight problems.
TV makes you eat more junk food. People who are watching TV eat more than those who don't. It's a fact.
TV makes you antisocial. You are at home, zoned in, instead of talking with real people, face-to-face.
TV is stressful. The news and many shows are filled with stressful stories. Avoid these, and you may feel things are not so bad after all.
TV keeps you from doing other things. This is the big one for me. The average person watches something like four hours of TV every day. That is 28 hours a week or more than 1,400 hours a year. If we all just put that time in to something else (exercise, volunteering, talking with our children), think of what a different world it would be.
http://longevity.about.com/od/liveto100/ss/life-expectancy_9.htm