I'm wondering about the practical outer limits of what drupal can handle. I have a client who currently has a site with about 40k regular users, and wants to blog-enable them.
It seems that the very promising Google module's development is dead. The module was build directly after Google API's were opened and was - at that time - state of the art.
In busy forums, moderation can be a useful tool for gently enforcing good behavior. Drupal is pretty flexible in its support for moderation, but it could go farther to make moderation more effective.