I find the Graduate Research Plan Statement to be the most daunting aspect of the NSF GRF application. The GRF program is an early stage fellowship, designed for graduate students in their first years, or who have not even started yet. How then, I wonder, can such students construct a statement detailing “what [they] want to do, why they want to do it, how they plan to do it, how they will know if they succeed, and what benefits could accrue if the project is successful”? Perhaps in order to compose this statement I will need to know in which laboratory and on which project I will be doing my thesis research – yet the application is due in my first semester, before I will necessarily have answers to those questions.
If I were to consider the Research Plan Statement I would want to read, as a member of a review committee for such an early-stage fellowship, I would want to see that the student has a clear idea of how to go about his research. Even if the student did not know exactly what his project will be or what form the research will take, I would want to see that he has a clear idea of the frontiers in his field. I want to see that he knows what has been done, what is still too far off, and where he could conceivably contribute to his field. Here, the student could show me that he is willing, interested and capable of furthering research in his field.
That being said, I still see a student with a clear research plan, who knows all the whats, whys and hows of his research, having a sizable advantage over the student I described in the previous paragraph. Between the two, I would select the student with a clear research plan.
References:  NSF GRFP Program Solicitation (2014)