The two elements that made "The Best Job in the World" such a massively successful campaign are its spread and its uniqueness.
As the case study elaborates, the news was everywhere, from print media ("in classified and employment sections of the newspapers"), news (reported by major news channels) and online (Youtube, Facebook, Twitter, etc.). The case study also presents statistics on this widespread campaign, with high "time-on-site" and large numbers of unique visitors and page visits.
The spread was even greater thanks to the "word-of-mouth" campaign which was a result of campaign's uniqueness. As stated, it showed the profile of the caretaker's job with Hamilton Islands in the backdrop. While it seemed to promote the job, it was actually the beautiful island that was being promoted.
So, a well-planned campaign became a great success due to its carefully designed attractive strategies.