What turns me on about hair is texture first. If the hair is super soft then that could be all I need. Even if it's not super thick, as long as it's soft and silky, it will turn me the fuck on.
That being said, thickness definitely helps. My most recent ex had the thickest hair ever. Unfortunately she had dyed it (she was Filipino) and that was a turn off because I loved the idea of my cock in her naturally dark hair. Hair color as long as it's natural is attractive to me. Colored hair can be too, but it will get to me every now and then.
But back to the thickness, thick hair is incredible and definitely what I look for next. Doesn't matter the length so much, as long as it's thick and soft.
Style also definitely helps. Some girls have beautiful bone straight hair that you can glide your fingers through, and then others have a sexy wave that just makes the hair bounce as it moves. And then there's short hair that just seems to constantly move and even if you mess with it it goes right back into place.
I know this is a long hair forum, and I definitely enjoy long hair, but let me tell you, short hair has its perks. My ex started with long hair but then cut it off to get rid of the dyed part. She went to this sexy chin length bob, and holy crap, feeling her shake it back and forth brushing over my cock was incredible! I had a friend with benefits type deal (who was Korean) after my ex and she had long hair. She did the same thing and it felt amazing, but not as good as the short hair. Course, that meant that with her long hair she could wrap it around me and brush it over my body as she did other things, but both have their benefits.