Great post, Patrick.
I was also late to Gangnam Style, by the way. :)
And I agree that it's one of those anomalies. However, I think what made it go viral was not its mass appeal, but the fact that it was different. The idiosycratic nature of this music video provided space for parody and piggybacking, which was what many people did.
For example, a similar thing happened to Macarena in the 1990's. Michel Telo was trying to repeat the success of Macarena with its Ai Se Eu Te Pego, but I don't think that choreography went global this time, if only the song. Which means that you can pull off certain tricks only once.