(1) Some concepts are "big concepts," in which case the main should give a high-level definition and a sweeping overview that points people towards many other concepts (like group theory and logarithm in the pages linked above). These pages should probably have a number of other lenses on them, some of which are the beginnings of paths.
(2) Some pages are "normal sized concepts," in which case the main page should define the whole concept (in a fairly neutral way that a fairly technical reader can understand), with other lenses that explain the concept to different audiences and provide other stuff like examples and exercises (I tried to do this with associative_operation.html)
(3) Some pages are part of paths, which are sequential posts that spend a whole series introducing a number of different aspects of a big concept; I'm currently putting together a logarithm sequence that demonstrates this. (The main posts are done, but I haven't strung them together yet.)
~ Nate