Hair is a tricky thing. It can be tricky to draw in 2D, but in 3D? Way harder. And animating it in 3D? Super-duper hard.
It's come a long way though. I remember when it was a huge deal when Pixar's Monster Inc was courageous enough to feature a main character covered in fur. But Disney's Tangled, based on the story of Rapunzel, takes it way further, featuring a character with long (LONG), beautiful, flowing hair.
This snippet from the always-awesome Science Friday gives a little insight into the process of creating that awesome hair.
I should probably go see more movies.