For Design Development Plan (DDP), I think the two most important parts are quality management plan and scope management plan. Because the quality management plan will tell you how to evaluate the difference between the actual outcome and the desired outcome, so as to provide the right direction to apply changes and make the project back on track. On the other hand, the scope management plan focuses on something bigger such as whether this project really have that potential market to sell, and how this key feature of the project will open up a new market, to avoid the overall direction to which the project is advancing towards is wrong.