To me, it's the little articulations that really make a fretless stand out. Take a line you normally play, and try this: for a note that is the resolution of a phrase or line, hit the note but immediately slide up a whole step (or half-step), then release it and play the original note. Also, sliding into notes at the right moment, even just a half step (especially if that note is not in the "key"), can add some very nice tension & resolution to an otherwise routine bass line.
Little articulations like this can be beautiful on fretless. Think of how good singers move around, over, and under melody notes to add interest to a line. Used tastefully, at the right moments, it will never be considered overplaying. I think of fretless as my most vocal instrument, so why not think like a singer?
(Jaco liked Frank Sinatra, Marcus likes Stevie Wonder...)