> Closely followed by the interactive notebooks.

Mathematica's notebooks are the only environment where I can do some computation to arrive at a symbolic expression. Copy the expression from the output cell into a new input cell. Then manipulate it by hand into the form I want. Then continue processing it further.

Also, symbolic expressions can be written nicely with actual superscripts and subscripts, and with non-latin characters.

One of the best features of Mathematica system.