So I've been digging into the sources these days and it was a pleasant experience.
But IMHO, the database schema could be improved:
Why is there no Foreign Keys ? Besides the delete/updates etc constraints, you would get performance improvements as you may know.
Query Joins are not costly so why the Denormalization, for the Discussion table particulary ? comments_count, start_time & start_user_id could be retrieved via the Posts table.
