Perhaps the most important defining characteristic of Lisp is "Code as Data." You won't get that experience in quite the same way with any other language. In C#, the closest analogue is expression trees.
It is that quality that makes Lisp an excellent language for parsing. It's also the quality that motivated Paul Graham to say of Lisp: "The unusual thing about Lisp-- in fact, the defining quality of Lisp-- is that it can be written in itself." Although self-hosting compilers are nothing new, no language does it quite as elegantly as Lisp does.
Metaprogramming (something forin which Lisp also excels) is also a worthwhile thing to learn.
Beating the Averages by Paul Graham
http://www.paulgraham.com/avg.html