Task Categories 1) Coding 2) Documentation 3) Marketing / Outreach 4) Code Tests 5) User Interface Improvements
Coding tasks are separated into either Platform or CMS tasks. When writing any code targeted for Joomla, it is important to follow all of Joomla's coding standards. Code will not be accepted or merged without tests and proper code style. There is information on this docs site and on Github regarding the code style requirements.
Documentation tasks consists of writing documentation here on the Joomla wiki.
-Help Screens --Document all Joomla 3.x help screens.
Joomla has a marketing team that involves it's self with performing various tasks such as usability of the Joomla.org family sites, SWOT analysis and more.
The platform and CMS need improvement in the test coverage. Tests written for specific packages that require more code coverage can be considered a task. See http://developer.joomla.org/coverage/ for specific coverage information.
Joomla has a JUX (Joomla User Experience) team that has done much of the work to improve the usability of Joomla 3.x. There is still much work that can be done to improve Joomla's usability and accessibility.