Just a couple of things to add to anyone wanting to write for harp.
1. If you want a bass note to sustain throughout a bar, you don’t need to have repeated quarter notes; the harp will continue to resonate until you dampen the strings. If you repeat or play a lot of notes in very low octaves (particularly on pedal harp) it will sound muddy.
2. Harpists like to “grab” eighth note groupings all at once; that is, we like to place all our fingers on the notes we will play before we play them. This means that arpeggios that are more than four notes (since we only play with four fingers on a hand) are cumbersome to play in one hand. It also means that asking us to play eighth notes and then suddenly throwing a chord on notes makes the passages that much harder to play, especially if the chord puts the hand into an awkward shape.
If I think of any more things I will be sure to post them.