Substitute teaching is a lot of emotional hard work. One cannot take things personally at all, one must always be mindful of what the teacher wants, and one must always refrain from judging.
When I go into a room to substitute teach I can feel the personality and teaching style of the teacher. The behavior of the children tell me what their routines are like and the level of chaos the teacher allows to dwell in the room. All learning takes place with some amount of chaos. In fact many times the more chaotic learning looks, the more real learning is going on for the younger children (although it must be within controllable amounts). The notes left by the teacher often tell my how much time out of hours the teacher spends working on her classroom. A workaholic teacher (the usual case) has a nice binder or much updated information for the substitute, however depending on life circumstances the notes can be non-existent.
I have learned so much about myself, teaching, and working in spur of the moment situations, I almost think that every teacher should spend time as a sub. I also see so many great ideas and activities that I would have missed out on learning about if I just had my own room. I wish I had time to learn more about these ideas with the teacher who created them, but alas when I am in, the teacher is always out.
I learned that I like to be the boss, and as a sub, that is not always (if ever) going to happen. I learned that I definitely prefer younger children. I have learned that I want to be organized as a teacher. One thing that I love about being a sub is that I get to go home and relax without worrying about planning. I also learned that I do actually love to plan, and collaborate with other teachers, and this is missing from substituting.
All in all, I highly suggest being a substitute teacher for at least a while; there is so much to learn about how to be a teacher.