When I was a student and working with Mathematica 5 and 6 a lot, I was always amazed about its simplicity and power. I had just two windows: typing in a white document window without any toolbars (only zoom control in the bottom) and having another window with documentation. And the documentation was great: you could evaluate examples right inside the doc page, play with parameters, like in your own document.

The design of the language, user interface and depth and clarity of documentation makes me think that Wolfram Research is even more design-obsessive than Apple. Stephen Wolfram takes credit for too many things (like Jobs), but he also delivers amazing stuff.