Learn how to discover the data behind a content-management system and generate the code to interact with the CMS API to get and modify that data. All following a platform-independent modeling approach
5 people are responsible for more than 50% of all commits in WordPress. Read this & other metrics giving you a better picture of how the WP code has evolved
Have you done any research work using WordPress as case study? Or using any of the technologies that WP users (PHP, SVN, DBs,…). This will interest you.
On the slow evolution of the PHP language (at the very least at the eyes of an “outsider” to the PHP community)