This is somewhat of a big task, but the module developer documentation for Drupal 7 needs to be reviewed, including the API section, to see if it needs updates for Drupal 7.
There are already specific issues for the Form API, Field API, and File API. I personally used the Database API section extensively recently, and I think it's fine. But, I'm not sure about the other developer documentation sections that are in the module developers guide and API guide:
I'm not as worried about the doc on api.drupal.org -- that is in better shape (and issues on that are in the Drupal core issue queue anyway). This concerns the doc on drupal.org that programmers use to create modules.