I've been at a number of failed startups. Here are some of the things that hurt or eventually killed us. Note that these can apply to projects within large companies as well. YMMV.
* We changed business models so our customers became competitors. How many ways do you depend on your customers? Is it just sales, or is it marketing too? Will your new customers really be able to replace your old ones? What is the transition plan?
* We always delivered what we promised and sometimes more, but late. Our competitors under-delivered but always on time. How sensitive are your customers to features vs timelines? Have you separated your features so you can deliver _something_ on time? Or is it all-or-nothing?
* The parent company/major investor had a bad year and shut down our funding. They were so big, that had been unthinkable. How diverse are your funding sources?
* After shutting down, we stayed together trying to resell what we had developed. We found an apparently interested large company and really tried to get them to fund our continuation. They were just playing us along, trying to learn what we knew about the market, then they released a competitive product that was inferior to what we had in development before the shutdown. How much do you know that your investors don't? Are you sure you want to tell them?
* The parent company/major investor decided our project was actually really cool and decided to move management back to the mothership, where they were pretty clueless about our market. That killed morale in our office (because it wasn't our project anymore) and slowed down decision making immensely (mothership was overseas). How much do you know that your investors don't? Do they appreciate that?
* We gave away too much for free. The freetards complained the most and required the most customer support and education. Very few of them converted to paying customers. When will a customer be forced to decide if your product is right for them? Why will they leave your free plan? Is your free plan free for you?
* Those that did convert only stayed with us through the middle part of their growth cycle. Once they got big enough, they also had enough money to hire an internal team to perform our service for them, believing that this would cost no more and lead to more control. Are your customers going to grow with you and are you going to grow with them? How? Why won't they leave when they get big enough?
* We spent our balance sheet on lawsuits instead of marketing or product development, defending a feature customers liked but was not core to our business. Why are you in court instead of on shelves?
* We saw the coming trend to run application software in a browser, 20 years too early. We based our product on unproven technology which we had no ability to improve ourselves (included hardware and software), and our prices were not competitive with the current technology. Despite this, we did have positive press and some sales. What, exactly are you selling, and why would anyone buy it? Can you find a practical compromise between your big vision and what people want to buy now?
* We were one company with two orthogonal products. One product was easy to understand and visually attractive -- it got all the press. The other was invisible, extremely powerful, but hard to explain to a broad audience. This made the company feel unbalanced, with razzle-dazzle that didn't lead to sales of the more valuable product. How do your products support each other?