The differences are subtle, but less so than text typography. Notice where the note flag bars (I'm not really a musician) fall on the staff, the slants on the rising/falling sets of notes. In particular compare the 12th measure, 17th measure, and note the differences between the 15th/17th on both (small numbers at the top start of the line are measures). There are more subtle points like the distance between the slurs and the notes and the weighting of the page so that the left is blacker than the right.

I love LilyPond. It should be noted (no pun intended), that it is released under the GNU GPL license and works on a variety of platforms, including my platform of choice, Mac OS X.

The most cringeworthy difference I see is mostly in measure 17, because the bars alternate between being above and below, whereas the notes themselves are not that different. Next is measure 19, because the lack of slant on the bars is particularly glaring.

The interface may not be the best for musicians; it reminds me mostly of LaTeX. But, as you mentioned, it can handle extremely complicated notation and the output is very high quality. I'm currently (sort of) working on an optical music recognition program and eventually want it to emit LilyPond code.

line (measure?) 9 seems to be missing some numbers here and there, also some of the note groupings? are missing;
line 17 seems to be missing them as well....

something tells me this wouldn't work well in unicode....
http://www.jbrowse.com/text/unij.html

Having some meters high of notes at home, I really appreciate their work. The subtility of the differences is really what makes the difference between a good and a bad. Because what is really important is the easy tracking of the notes while you are completely into the flux of playing the music. At some point you do not even pay attention anymore to the notes, you just glance at them and you find the move where you are intuitively.

I really like this article, it reminds me of what we try to achieve when doing "good design".

Side note: Your comment system does not like non ascii characters.

I love that they're so fanatical about it even though for the most part I don't notice the difference. Personally though i don't like, in general, that they let measures expand or contract as needed. To me a wider measure feels as if it should take more time even though that's totally untrue. To me sheet music is more like a graph of pitches over time.