I wish Emacs would provide a canvas like API to draw in buffers. That way integrated graphics could be easily added. This would create a completely new Emacs experience.
And arbitrary objects can take the role of unique values. It's quite convenient, especially considering that Mathematica's language and display system let you put arbitrary expressions anywhere you want (like a 3D plot animation used as the surface texture of a 3D cube).
[1]: http://racket-lang.org