The learning curve for Lilypond is really no worse than for any other text-based music typesetting software -- and far easier than, say, the TeX music packages. ABC is a little more simple on the surface, but is so much less flexible as to make it useless for complicated music. MusicXML seems to be a good transport format, but as parent points out, it's clunky: like any other XML format, everything is perfectly readable and will take a year and a day to type.
...wait, they'll give handj-- I mean, I can go to Starbucks on these asteroids? Sign me up!