So first, maybe I should describe more about how I did my application, and then I'll answer specific questions.
I applied to the NSF GRFP in November 2009, the same time I was applying to graduate programs. I was doing research as an undergrad, so I worked with my adviser at the time on my research proposal. The research proposal was an extension of what I was doing at the time. So it was original, but it was a logical extension of our work, so it wasn't too original. Below are the intellectual merit criteria and you can see that they focus on new research that is doable.
- How important is the proposed activity to advancing knowledge and understanding within its own field or across different fields?
- How well qualified is the proposer (individual or team) to conduct the project? (If appropriate, the reviewer will comment on the quality of prior work.)
- To what extent does the proposed activity suggest and explore creative, original, or potentially transformative concepts?
- How well conceived and organized is the proposed activity?
- Is there sufficient access to resources?
Broader impacts is more difficult to address. But here are the actual criteria.
- Broader Impacts* – Activities and projects that:
- How well does the activity advance discovery and understanding while promoting teaching, training, and learning?
- How well does the proposed activity broaden the participation of underrepresented groups (e.g., gender, ethnicity, disability, geographic, etc.)?
- To what extent will it enhance the infrastructure for research and education, such as facilities, instrumentation, networks, and partnerships?
- Will the results be disseminated broadly to enhance scientific and technological understanding?
- What may be the benefits of the proposed activity to society?
So it is an advantage if you have done some teaching / tutoring previously, but it is not a requirement. The last three criteria can all be satisfied by research that recognizes its own place in society, meaning, that one can explain that this research will have a broader impact outside of this one project.
Here is roughly how I think I devoted my essays towards each criteria:
Personal Statement: Intellectual Merit 50%, Broader Impacts 50%
Previous Research: Intellectual Merit 90%, Broader Impacts 10%
Research Proposal: Intellectual Merit 75%, Broader Impacts 25%
So for me, it was definitely tougher to spend a lot of time addressing Broader Impacts, but it is important to address it, even if only a little, in every essay.
I should note that since getting the NSF GRFP I have changed research interests twice (still within physics) and that has been no issue. As others have put very well, the NSF GRFP funds a person, not a research proposal.