I believe mental toughness plays a role, as does self belief and confidence. There are those that will argue winning breeds winning, while others believe that talent breeds winning. More often than not, the team with the best overall talent will win. It's why the West Indies dominated the World Cups throughout the 70s and Australia since 1999.
In saying that, there is value (albeit unquantifiable) in the experience of having been there, done it and knowing what it takes to win.
Sport is made up of two components: physical and mental. The South African teams have been very strong in terms of their playing abilities but have shown mental weakness and at times sheer panic.
What hasn't done them any favours over the years is the huge amount of pressure weighing on their shoulders due to their past performances in the knockout stages. Prior to the last World Cup, they had yet to win a knockout match. Knowing that and having that rammed down your throat in the pre-match build up adds pressure and doubt, and erodes confidence. Some players try too hard, others panic, some over-think, some forget to think. Pressure effects each player differently. What separates a great player from an elite player? The ability to handle and perforum under pressure.
Malcolm Gladwell wrote the most eloquent of articles regarding the art of falure:
gladwell.com/the-art-of-failure/ 
