I never can understand the obsession with the Two Slits Experiment. Diffraction happens everywhere. All the arguments rehearsed here apply in every em interaction in the Universe. People seem to want photons to be different when in a very simplified context.

I think it's a beautiful experiment because it was so useful in overturning previously held beliefs and contains an incredible amount of physics. The problem is that people assume that the 2-slit experiment is just one experiment that always shows wave/particle duality. All it really does is show that whatever forms the interference pattern behaves as a wave. This has been a huge step at different points in the history of physics, however, and is still one of the most dramatic ways to demonstrate wave behavior. (I'm biased here, since I do work in coherence theory, but its still a nice experiment!)

1) Classical light: The 2-pinhole experiment verifies that light is indeed a wave since it shows interference patterns. You can also investigate the coherence properties of light (i.e. how much randomness is involved--varying from very random/incoherent for black bodies to very ordered/coherent for lasers).

2) Relativistic quantum mechanics: You can explain what's going on by field theory/path integrals. The particle takes all possible paths from each slit, but because of the mathematics involved in QM, only some of the paths end up being useful. There's a great deal of similarity between this and the classical waves, because the mathematics of QM wavefunctions and the mathematics of classical waves end up being similar: i.e. waves can cancel each other out just like the probability wavefunctions. This does show wave/particle duality if you use relativistic electrons, which we already knew behave as particles in many applications.

But I agree with sophiecentaur. The strength of the experiment is in demonstrating that whatever you're shooting at it is a wave. Since most of us would have no doubt about the wave nature of photons (they make up light waves, after all), watching them behave as waves isn't really a surprise... For photons, what is interesting is watching contexts (the photoelectric effect) there they behave as particles rather than waves.