More on concrete ideas while talking about them in The English Language and Your Ideas:
It’s not novel that success is in the execution not the idea. Proper execution comes from turning the abstract into concrete and being able to form the imagery of an idea into something “actionable”—then doing that. Developing ideas in written word can make you familiar with the process needed to complete other projects. Both start with creating a concrete idea, or in GTD terms, determining the desired result and then working from there. Without concrete ideas, you will struggle writing the next paragraph (or even the first one) or finding the next action for your project.
Embrace the constraints of writing to practice solidifying ideas into something concrete.