If I had to pick one field that I consider challenging for differentiation, I would pick reading. My reasoning behind this is based on my personal experience with reading. I never had a diagnosed reading disability, but I was always a slow reader and thus did not enjoy it that much. That is until I took a study skills and speed reading course in college, on the advice of my college guidance counselor. That course helped me tremendously in both general reading as well as taking control of my own learning.
These two posts offer a variety of tech tools to help differentiate reading instruction and learning.