vast wasteland = wide variety of garbage
Saying that television is a vast wasteland means that television is worthless and has no positive values.
Newton Minow does give you a clue as to what he means when he adds:
"You will see a procession of game shows, formula comedies about totally unbelievable families, blood and thunder, mayhem, violence, sadism, murder, western bad men, western good men, private eyes, gangsters, more violence, and cartoons. And endlessly commercials -- many screaming, cajoling, and offending. And most of all, boredom. True, you'll see a few things you will enjoy. But they will be very, very few."
Newton Minow does not claim that television is all bad. He does indicate there are some informative and entertaining programs on the air. However, he believes that when television is bad, there is nothing worse.