> Consider the transformations you might do on a piece of source code while you're playing around. A lot of these (for me, anyway) are things which change not just position or name, but the part-of-speech of words, or temporarily make a nonsensical expression. (I do it in my shell a lot, too.) Rigid structure-enforcing editors hinder this.

I hope that this can be mitigated by using an incremental parser (pretty easy to write for a lisp), which gracefully switches to a plaintext representation if the underlying code cannot be parsed anymore. The advantages of an editor which understands the code (both, syntax and simple semantics) you are writing or reading are just too promising.