If you write or transcribe music, you need to take a look at LilyPond. Think of it like LaTeX for music, only easier to write.
And what's more, the developers really seem to get it when it comes to power, flexibility, and producing quality results.
Personally, I like immediate visual feedback, and I like direct manipulation, but I also like access to underlying structural information. That's one reason I like HTML for writing: It's easy to write sensibly, and you can customize it enormously both directly in the markup and via stylesheets, and you can get feedback extremely quickly, possibly even interactively. I seem to recall a TeX system for NEXTSTEP with an interactive typographic-quality preview that many academics used...
Thanks to Stellman for the links to LilyPond.
