Phew! I've just finished writing up a doc giving a rough overview of where we're going with the depsgraph design, and the types of things it aims to do (i.e. problems it should solve, etc.). There may be a few things I've glossed over here and there (and it may ramble a bit at the start a bit), but it should go a long way towards explaining what's going on.
Check out (and join) the discussion and document (check the attachment there) over on the bf-animsys mailing list: