I believe that to be a successful mathematics teacher then you need to teach all subjects in the discipline. I find that many mathematics teachers will become an expert in one area and stick with that area and only be comfortable teaching in that area. I believe that this is not the right thought process for an educator to make. I believe that to successfully teach students you need to know where they are coming from and where they are going. If you have only taught high level math classes then you are not sure about the basics that your students are coming in with, and if you don't know where they are going then you are not sure what needs to be emphasized for future success.
Along with teaching all subjects within the discipline, I feel that having a team teach teaching experience allows you to reflect on what and how you are teaching. This is my first year teaming and it is an experience that has opened my eyes to what/how students think. My AP Calculus students advocate for themselves and ask thought provoking questions, teaming students will slide back in their chairs and you as a teacher will have to pull questions out of them.
Final Thought: In my opinion to be a successful mathematics teacher you need to be flexible, driven and open to new ideas. It is funny because I feel those are the same things we ask from our students.