The Game of thrones series built its character slowly. Some people have only seen the start of the show and evaluated the series from that. They have not seen the entire series and so they think that the fans are overrating a show.
As a matter of fact, the point where fans started calling the show the best there is, the non-fans started lashing out and detesting the series even more. The cast is great, the plot is captivating and the performances are extraordinary. Once a person sits through all the seasons, his evaluation of the entire season will change.
The reason that a lot of people believe that Game of Thones is the most overrated program in television history is that they most likely never watched the show. Then there are those out there who watched the early episodes of the series and believed that it was too gory and vile and that they could only say negative things about the show.
The hype was strong behind the show but there were a very good amount of the overall viewing public who could care less about Game of Thrones or Westeros or Essos or The Wall. Game of Thones is certainly a cult classic and if you were not in the cult then you most likely had nothing or negative things to say about it.
The push behind it being the greatest show of all time has also led to people saying that Game of Thrones is overrated. Saying that Game of Thrones is the most overrated show all of time could be debated but there certainly is some people out there who would try and make a convincing argument.