What makes Lisp unique, in my opinion, is its elegant unification of code and data. The 'eval' function and of course Lisp macros are very powerful features that come out of this immediately, and theoretically you can implement any other language feature on top.

In most other languages, to do the same things you have to parse the syntax, build an AST (Abstract Syntax Tree), do whatever transformations you need, and if you're very lucky you can then call the compiler as a library. Lisp makes the AST directly executable with 'eval', and provides a syntax parser already. Moreover, the AST is made of lists and symbols, which Lisp is naturally perfectly suited to manipulating.

All this makes Lisp quite beautiful as a mathematical object. I can't speak on its practicality ;)