I dream of a world where people don't compulsively fantasize that for any given piece of software, it should be supplanted by some other piece of software whose main differentiating feature is that it's a FOSS clone.
Mathematica being open source would mean that I could look at the source though and see what the hell is going on inside. I'd love to see how they are doing call outs to R and trace through all the interactions between the kernel, the user interface, and W|A. If it were open source, I could see the implementations of all the Mathematical algorithms and look at all the neat optimizations and new results they have found during their work. I care about the "math in Mathematica and am interested in seeing how it all works.