This is what's missing from LilyPond....

[Felipe Gasper]

...which is also missing from Finale and Sibelius. Or at least, what would
make me find the software more appealing. :-)

Fretboard diagrams. Sibelius's support for fretboards is crude at best.
Finale's is much more versatile, but it's still clunky. Given the demand
for notating popular music, I would think fretboard diagram notation would
be a priority for Lilypond. As a suggestion, look at Finale's options, and
try to make it easier to notate capoed guitar chords, as well as being
easier to have a library of custom chord voicings, tunings, etc. See the
guitar edition of the folk hymnal "Gather" by GIA for reference, widely
used in American Catholic parishes.

Further along the lines of popular music, I would add good, robust
support for guitar tablature. While I will maintain that guitarists
*should* learn to read notes, tablature is used very widely in popular
music, so much so that it's become a standard feature of notation
software.