Going from BC to SL is the hardest hurdle to me. The reason is simple, I can't wear a bun, so my hair is unprotected, if I gel my sides down it could cause traction alopecia, and the SSK's war is on. Once I get to full shoulder length, I can place my hair into a loose bun, and have gotten to BSB in nine months.
Of course for those going for classic, that must be tough because we still don't know how many of us have hair that will grow that long, or what hair styling sacrifices have to made to achieve healthy classic length hair.