The biggest thing I learned was being PIC. You have to decide when they're doing the right thing, if they're going to continue doing the right thing. Do all of this while anticipating their next move, ATC's next move, what sort of dangers it could put you into, and when to step in. I think you learn more as an instructor than you ever will as a student.

With all of that, I enjoyed primary students the most. Unless you have somebody with an incredibly hazardous attitude, they don't do as much stupid stuff that can kill you because they don't know enough about it yet. My worst experiences as an instructor both came while working with students on an instrument rating. They had bad habits from past instructors and in one case I stopped and went back to basic fundamental stuff because his airmanship was so poor.

At 80 hours we think we know everything. By the time I had my CFI at 260 hours I was amazed at how much I didn't know. At 500 hours it was amazing how much more I knew than I did at 250, but how much I still had to learn. At 1000, you get the idea. We'll see how I feel at 5000