What does it mean to be a 21st century teacher?

A 21st century teacher needs to have an in depth knowledge of the subject they are teaching. Research has shown that teachers who are considered specialists in particular areas convey it to students to a greater degree than those who do not. Yet in the elementary setting there seems to be a push toward self contained classrooms where the teacher is expected to be an expert in all subject areas. This is rarely the case. We end up with "Jack of All Trades" teachers instead of specialists.