Dan Meyer seems to have come across a recent theme in his lessons: he's not giving them the answers. I think that's awesome.
This is something you could very well expect to see in any art class that has a good teacher, since in art you will often find three or four (or more) opinions about composition, color choices, techniques, or even the definition of "art" itself. We can't give a single answer because there isn't one. We're kind of forced into the scenario of not giving an answer at the end of the lesson that students can expect to see on the final.
Mr. Meyer, however, is a math teacher. There are very concrete answers that can be figured out when encountering math problems. Two plus two has an answer. If solving for X yields more than one possibility, you can graph them to show the whole range.
Sometimes the conversation, the act of sorting things out with your peers and learning for yourselves what the right questions are is more important than coming to a specific teacher-sanctioned conclusion.
Giving an answer at the end can disguise that simple fact. I'm glad he's realized this so early - I know more than one teacher twice his age who's yet to have that sink in.