Mathematics education is an academic field in its own right, distinct from mathematics though obviously related to it, in much the same way that computer science was 20 years ago. When I was an undergraduate in the mid 1990's, my university did not technically have a computer science major, only a computer science specialization within mathematics, and many computer scientists were technically part of mathematics departments.
Something similar is happening with the field of mathematics education now: a large number of researchers self-identify as specializing in mathematics education. Some of them work in mathematics departments, but an increasing number go to their own departments and programs: e.g. at UGA there is a department of mathematics and science education, within which there is a program in mathematics education (in fact one of the top ranked ones in the United States).
A site for mathematics education thus includes an entire academic discipline, but also a much broader audience of K-12 mathematics teachers: this is quite a large group. This group also belongs to the much larger group of all educators...just as people in various academic disciplines all belong to academia and intellectual discourse. Such commonalities are certainly nontrivial, resulting e.g. in the fora on the Chronicle of Higher Education. But for a focused Q&A site, it seems much more uesful to focus on the community of all mathematics educators. This is indeed a natural community which has already self-organized in various "horizontal" ways (e.g. NCTM) and desires to integrate itself more vertically.
In terms of both clientele, content and goals this is overlapping but highly distinct from both MO and math.SE. I think that anyone who regularly participated on either or both of these sites would find this easy to understand and appreciate. From far away, sure, it sounds like "math math math"...but by the way that's how a lot of the various computer-focused sites sound to me. At some point perhaps I will learn more. Until then, live and let live...