In education, teachers start most of the times with a completely ignorant audience. In mathematics we start with the basics and slowly, slowly, build our story from the ground up towards higher algebra and advanced geometry.
Many presenters use this mindset when pitching a business idea and/or investment. Their presentations resemble a 101 text book on the market.
But in most cases, your audience will know quite a lot. Venture capitalists see dozens of pitches of internet-related businesses every day. Surgeons get pitched everyday with new drugs and medical devices.
Think about that when designing your pitch. What thought process would an intelligent, knowledgeable person go through when confronted with my idea? First she needs to understand what it actually is. In her mind she probably tries to compare it to a concept she already knows. Then she probably starts asking herself the big obvious questions. Does the world need this? Why is this so special / has no one else done this before? Can these guys pull it of?
Do not try to educate your audience, instead try to answer the obvious questions she cannot answer herself.