Founder at NailMyStartup.com - Hands-on Startup Founding Team Advisor, Strategist, Accountability Partner, Tactical implementer, Mindset expander, Sounding board and Sanity Check.
If you're lucky you'll get 20 minutes to pitch your business to get the funding you need to grow. First impressions matter and the first 20 seconds of your first impression matters most. Learn how to excite investors and present what matters most to them in a way that positions your value proposition as a viable, scalable, fundable opportunity.
Working with over 200 entrepreneurs and founding teams I've found the lean canvas business model methodology has proven to deliver the highest ratio of success. Customer development works in parallel with product development quickly and methodically to get to product-market-message fit before the money runs out. This process will make sure you avoid the big and costly mistakes that derail most startups, even ones that originate with great ideas.
Many of the answers already provided are valid. Many startups fail because not enough people want, or will pay for, the product. Others fail because they are unable to generate enough revenue to sustain before they run out of money. In my experience I've also seen startups with great ideas and some traction (evidence of a product-market-message-fit) fail because the entrepreneur couldn't get out of his or her own way. Mindset is key. A willingness to iterate and pivot where necessary is key. A realization that you don't know what you don't know that you don't know as an entrepreneur - and the commitment to fill in the missing bits and pieces is essential.