it's a huuuuuuuuge amount of technology in the standard library of mathematica, beyond the surface syntax and rewrite system, i mean.

Now Mathematica notebooks (still remember, it is .nb) do not have the novelty factor. But they were the first to set a trend, which we now take for granted.

That said, I rarely use notebooks anymore. In the coding time, it is much easier to create scripts and ask to create a visualization in HTML.

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.

From a purely programming language theory, it's pretty unique.

I once tried to find a language that had all the same properties, and I failed. The Factor language is probably the closest. But they are still pretty different.