Just reading through the TdF thread, and I reckon cycling (road racing?) must be up there - lots of niche terminology, and when you watch it, it just looks like a load of guys cycling companionably together, seems very difficult for a novice (ie me) to work out any sense of tactics or competition.

American football is obviously more complex than a lot of sports, though once you get your head round the four downs system, it's easy to get the hang of.

Reckon gymnastics and diving etc must also be up there, almost impossible for the layman to spot the fractional differences between gold and silver standard competitors.