Hi, I appreciate that this idea is being discussed, and here are my thoughts, for what they are worth!
First, the Demonstrations site is great, I appreciate being able to search by topic, and how the demonstrations have a clear focus so you can zero in on a particular topic.
I REALLY appreciate the fact that it is "open source", I have learned so much and gain a great deal by seeing the work that others have done.
My wish for a "Lessons" site would be something similar, except to have a place where Mathematica users could post actual lessons on particular topics.
These would very likely contain demonstrations, but "fleshed out" with notes, examples, sample problems, etc.
For me personally, I would like these to be specific and limited in scope. I completely understand that many people would like to see an entire course, but , as I said, for me, that would be difficult. Many teachers are working with specific programs of study, at least here in Alberta we sure are, and I need specific lessons on specific topics.
It would be great to be able to search for, as an example, Conics and find several lessons dealing with conics, parabolas, hyperbolas, general form, standard form, degenerate, etc. etc. etc. Each lesson might have a few demonstrations to help with the teaching, some teaching notes, sample problems and solutions, etc.
As more and more courses are delivered online, and as more students have lap tops or access to computers at school/home, a repository of GOOD lessons would be so useful.
I respect all the work textbook publishers have done, but for me personally, as an online teacher, textbooks are quite limited. They don't have enough examples, solution manuals and teaching notes come bundled in "add on" bits and pieces, and it's challenging to put together a lesson that is cohesive and works for a student trying to learn mostly on their own.
I realize other teachers don't want or need that level of information, I'm just sharing my wish list.
I think if the lesson content was specific, format (I.E. stylesheets, etc) could be flexible. Just as with demonstrations, you can get the "source" and modify as needed.
I'm rambling, so I will stop. I am very interested to see where this could lead.
I can only imagine the possibilities if 10 - 15 teachers all brought their collective energy and knowledge to bear on a particular topic in math, and created lesson resources that could be of great benefit to ourselves and our students.