1. WordPress database and the API – access and manipulate data, handle user roles and permissions and posts.
2. Hook into the rest of WordPress using actions and filters.
3. Change the way your WordPress backend looks by customizing menus, submenus, and the plug-in admin panel.
4. Integrate AJAX and jQuery into your plug-ins to dynamically generate content
5. Hook directly to WordPress edit pages and use AJAX to generate fast searches
6. Integrate your plug-in with WordPress panels and the tinyMCE editor
7. Access and work with third-party APIs like Flickr
8. Implement localization support for users of other languages
9. Maintain and manage your plug-in using SVN and publish it to the WordPress Plugin Repository
10. Handle security issues and make your plug-ins safer to use