Structure in the Classroom

Thinking back to the first day when we drew our ideal classrooms, I remember discussing structure as being in my dream classroom...obviously structure is something that is important to me.
Structure is important to me for a lot of reasons. It benefits the students because it allows them to really know their environment, therefore allowing for them to hold expectations. Hopefully, the knowledge, experience, and expectations they gain from this structure produces a safe and open environment between students and with the teacher.
Structure also benefits the teacher. We all posses strengths and weaknesses in our skill set, and I think things such as structured, thought out, detailed lesson plans can help the teacher feel secure in some of those skill areas that are lacking in comparison to the others (certainly this is not to say we shouldn't prepare in our comfortable skill areas). I think structure can be a teacher's friend in fortifying ideas, concepts, and rules. This could exist, for example, in the fortification of skills we have learned by doing multiple short activities over a period of time focusing specifically on those recently learned skills, or (unfortunately) in our classroom/school disciplinary structure.
I would agree with many of colleagues in the thought that teaching is improvisation and that flexibility in a classroom is so very valuable. However, even improvisation usually happens within some constructs - and those constructs (structures) are going to be the foundation for classroom operations.