In my opinion, it's the fact that it's a regular show and has elements that are risque. As with anything that is well regarded, a good amount of marketing went into this series. I haven't looked into it, but I've heard of a lot of people doing so just because they saw the clothing or posters for it. Because of social media as well, it's one of those shows people just enjoy talking about to death and so it gets showed to people that otherwise wouldn't be interested, who want to keep up with their friends.
Television shows, books, and any medium can be addictive when you have the right elements in place. I believe that things are okay in moderation, but when you put out a show or a series of books regularly, people tend to get overly excited and into it even if it's not that great all the time. It happens time and again, like with Harry Potter.
Here's an interesting infographic on addiction to social media to show that when the reward centers of our brains are activated, we have trouble slowing down.
1 - http://www.allaboutlifechallenges.org/television-addiction.htm
2 - http://usatoday30.usatoday.com/life/books/news/2007-07-16-potter-phenom_N.htm