I just started exploring fountain pens (inspired by a previous HN posting), and the feature that appeals to me the most is the variety of ink colors available for fountain pens. Most ballpoint pens seem to have just a handful of ink colors, but it seems to be possible to get a lot more colors for fountain pens, and some of the colors are really exotic.
Also fountain pens draw really fast with almost no pressure and have such a consistent line. I've been making little musical drawings with them. https://tiktok.com/@whistlegraph