In brief, Lisp's S-expressions (the parentheses) are a straightforward instantiation of the abstract syntax trees to which all other languages parse.
You can manipulate that tree easily and directly in Lisp, whereas other languages (that are not variations of Lisp) hide parts of it, to varying degrees.
All other languages are either equivalent to Lisp, perhaps with a different superficial syntax (there are very few of these; one is Mathematica) or expose only a strict subset of this capability (most other non-Lisp languages.)
In particular, Lisp makes it trivial to do (real) metaprogramming -- programs that write other programs, including the introduction of entirely novel syntactic constructs -- via macros. Lisp macros can create new language syntax that controls when and how code is evaluated, perhaps rewriting it.
Non-Lisp languages can't do that fully, only subsets of it. Ruby metaprogramming, for example, is clever syntactic sugar. You can give methods clever names and use clever calling conventions so that they read more similarly to English, which is called "metaprogramming" in Ruby; but you don't have universal control of the parsers from the level of ordinary Ruby code, and you can't completely control when and how code evaluation happens.
Now, how practical or useful is all that? Opinions vary, from hardcore "you should invent a full DSL with unique syntax for each particular application" Lispers, all the way to those who say macros are a fun but dangerous toy with limited practical value.