Could you give more examples about things you like about Mathematica?

1) Mathematica's programming language does not confine you to a particular style of thinking. If you are a Lisp fancier, you can write entirely Lispy code. Likewise Haskell. There is even a capability for relatively painless dataflow programming.

2) Wolfram Inc. took great pains to make interfacing with the outside world from within the app as seamless as possible. For example, you can suck in a spreadsheet file directly into a multidimensional array. There is import and export capability for hundreds of formats, including obscure scientific and engineering ones. In case the built-in formats do not suffice, defining custom ones is surprisingly easy.

3) A non-headache-inducing replacement for regular expressions. Enough said.

4) Graphical objects (likewise audio and other streams) are first-class data types. They are able to appear as both the inputs and outputs of functions.

5) Lastly, and most importantly: fully interactive program development. The rest of the programming universe lives a life of endlessly repeated "compile and pray" cycles. Mathematica permits you to meaningfully evaluate and edit in place every line of code you write. I am otherwise an Emacs junkie, yet I have never felt the slightest desire to touch Emacs when working on Mathematica code. The programmer's traditional need to wade through and shovel giant piles of text from one place to another while writing code is almost entirely absent when working in this language.

The downsides of Mathematica (slow, proprietary, expensive, etc.) are widely known. Thus far, the advantages have vastly outweighed the problems for my particular kind of work. However, I have found that I now feel extremely confined when forced to work in any other programming language. Perhaps this risk should be added to the list of disadvantages.

I learned about Lisp after Mathematica, and was like, "wow, that must have been where Wolfram got the idea."

Wolfram had (at least in the early days of Mathematica) a very interesting relationship with Lisp. He seems to have initially rejected many of its ideas, but it is clear that they somehow crept back into his work as time went by.