Most important thing?
This is kind of a difficult question and I've been sitting on it for a couple of days. I thought of all sorts of things that I've observed, both good and bad, and the best advice I can give myself or any prospective teacher is to be Patient.
Patience is something that to me is the basis for all the other teaching skills that I've seen. Be patient. Step back and evaluate what is really going on. Is there a behavior issue? Is the child asking for clarification or asking what the deeper meaning of the lesson is? Are there extracurricular issues impeded or increasing a student's learning ability? Is there anything else that I can do to aid the learning process? Is there anything I can do to better control my classroom?
Be patient. School is hard. Life is hard. Students have so much going on that sometimes they can't be completely present. Be patient. Control, don't react. Be patient. Be an adult. In all things be patient and everything else will be easier sorted out.