There are many reasons that all states require teachers to be certified. Knowing your content well and the ability to teach students are not the same thing.
There are many,many layers of teaching! All are equally important:
1)classroom management (without which, no content will ever be taught)
2) knowledge of developmental levels (Vygotsky's ZPD - Zone of Proximal Development)
3) differentiated instruction
4)UbD - (Understanding By Design) - a framework that focuses on backward planning and Teaching For Understanding
5)SIOP strategies to address English Language Learners
6)extensive knowledge of the various stages of literacy development
7) data collection and the ability to use that data to guide instruction
8) critical in connecting all of the above is..... RESEARCHED BASED instructional strategies!
Sorry for the lengthy response, but the question was pretty loaded!