The main feature of music notation software should be the facility to produce high quality sheet music the easy way; enter music notes as simple as possible, using either PC-keys, mouse, or a MIDI-keyboard. Afterwards it should enable you to print your scores in any way you feel fit. Of course, extracting single parts from scores enhanced with chord symbols, lyrics and so on, should be a matter of a few keystrokes or mouse-clicks, without the need of updating the score itself. It would be really nice when keyed-in scores would sound just as written including grace-notes, repeat-signs, chords, dynamics or whatever, so the software could act as tutor/répétiteur.
If the score editor assists arranging by generating accompaniment parts from chord symbols and creating an additional voice as starting point for a real second voice, would that be regarded a nice thing. The ability of easy transposing is a prerequisite. Transposition of a single part or a complete score should be as easy as a finger snap ("let's play the Bb-clarinet part using a saxophone in Eb"). By the way, MIDI-facilities would come in handy, as well as percussion-notation. As for the matter of distribution of music, need for printing (full score, part extraction), graphics generation (BMP, JPG, PNG, PDF, Postscript), file import and export (MIDI, ABC, music-XML) come to mind.
Fortunately, all this is accomplished with the music score editor MusiCAD