Clojure (and ClojureScript) have stand alone compilers and ship very nice APIs to compile code (if you want to write your own compiler). People use lein (and its plugins) because mucking around with a compiler and then managing dependencies, setting up your classpath manually, and writing a test runner is a waste of time.
As far as text editors go, you're not forced into using an nrepl-enhanced client. I got by fine on tmux+vim with no editor support for clojure other than syntax highlighting and rainbow parens for a long time. That said, having paredit, nrepl (for code completion, dynamic eval in the correct context, documentation lookup, misc other plugins), and a test runner with easy macros in my "ide" makes my experience better. There's absolutely no reason why one couldn't just use a notepad and the clojure compiler plus whatever random build system strikes their fancy.
As far as text editors go, you're not forced into using an nrepl-enhanced client. I got by fine on tmux+vim with no editor support for clojure other than syntax highlighting and rainbow parens for a long time. That said, having paredit, nrepl (for code completion, dynamic eval in the correct context, documentation lookup, misc other plugins), and a test runner with easy macros in my "ide" makes my experience better. There's absolutely no reason why one couldn't just use a notepad and the clojure compiler plus whatever random build system strikes their fancy.