As you start building the product, don't assume that you know all the answers. Listen to the community and adapt. We had a lot of our own ideas about how the service would evolve. Coming from PayPal and eBay, we saw YouTube as a powerful way to add video to auctions, but we didn't see anyone using our product that way, so we didn't add features to support it.
Source: Business 2.0: How To Succceed: Give Your Startup a Fighting Chance: http://money.cnn.com/popups/2006/biz2/howtosucceed/6.html
Contributed by: ~C4Chaos