Children spent a lot of time in front of the television and that is not always a bad thing. There are quite a number of excellent educational programming that can help them develop better reading, writing, comprehension, math and science skills. Even pop culture entertainment has value if it offers good lessons on moral behavior.
The problem with kids watching too much television is when parents use the TV as a babysitter.
Unfortunately, there are parents that shirk their responsibilities and simply let their young ones sit hours and hours a day in front of the TV. The faulty logic at work here is the children will stay out of trouble because they are parked in front of the television.
The troubling issue here is children end up being denied interaction with friends and family because they are locked in front of the television for several hours a day. This can have a very troubling effect on a child's ability to develop social skills and interactions with other his/her own age.
While these problems might not manifest very early on, they will do so in time and to the youngster's detriment. Rather than allow such a situation to unfold, it would be much better to not let the TV act as a babysitter.