Gregory Maxwell wrote:
Saying Lilypond is hard is like saying wikitext is hard. Lilypond is just one step above ABC and much more expressive, anything else would not be sufficient to produce and maintain a professional quality score.
I haven't been heavily involved in the open-music community, but I've attended a few academic computer-music conferences on and off, and the people I've met are somewhat ambivalent about Lilypond. Basically, the main complaint is that it's intended to be a music-*presentation* language (or music *typesetting*), not a music-*representation* language. This makes it hard to use for a lot of things, like automatic manipulation, transformation to ther formats, and so on. It also makes it relatively hard to edit, since you have to specify a lot of the nitty-gritty presentation details yourself.
Of course, it's a poorly-funded area with only a relatively small body of interested people working on software, so there aren't many good replacements. There is one system called GUIDO, that was promising but still very early the last time I heard about it; it may have a release by now, but I don't know of it. One problem is that most people who are interested in this sort of functionality just give in and use Sibelius or Finale, both fairly full-featured but proprietary commercial programs---an "open source Finale" hasn't yet appeared.
There is also an MPEG working group trying to develop an open standard for music representation, but it works slowly. The main stumbling block, besides being a working group in the first place, is that music representation is a much harder problem than it seems at first---there are a *ton* of variations in how people have written scores over the years, many of them conveying substantive information that needs to be captured somehow.
-Mark