Great question! Many things turn me on so much about a woman's hair.
First is softness/texture. When my wife brushes her hair against my cheek, my face has a little orgasm. Feeling that elsewhere on my body is heavenly.
Length is also visually important, but at some point, how many times do you have to wrap hair around your cock to get that wonderful feeling? I suppose if she's going to drag her hair up and down your body, extra long hair would give you maximum coverage. On the other hand, you get more movement and sensation when her hair is not as long and heavy.
Thickness is important for feeling, but it can also make a woman attractive on a more basic level as an indication of good health and genes. That primal part of the brain that looks at every woman as a potential mating partner and mother responds well to women with good skin, teeth, nails and hair.
Finally, there's style. A sexy, wild hairstyle can also be very attractive, especially when it accentuates all the attributes listed above.