One of my advisors/letter writers has a lot of experience with the system, and says broader impacts is one of the most overlooked, yet important things. Basically everything is important, grades, research proposal, past research is good/papers.. but honestly, pretty much everyone has a decent track record for that if you're a PhD candidate in science/engineering. So the broader impacts is where you get to really set yourself apart, since many fail to realize it's importance. It helps to read winning essays for sure.. most labs should have access to some, so if you didn't get it, try next year and learn from those who did.