I greatly dislike it when they have physically impossible or impractical hair, like Jirayia, Madara or Mei. Hair doesn't just work that way, spiking in all directions and stuff, and it should be a pain for them to lug that mass around, let alone wash/brush it. Or even characters like Deidara who have ponytails randomly sticking out from the rest of hair mass. I don't dislike characters themselves because of that, but that means I have to do some brainstorming to come up with a realistic mental image of them.
Also Kushina supposedly hated her hair. Why didn't she cut it short then?