The number one thing is understanding what customers want. Every business guru and product person in the valley will tell you to "talk to your customers". The major exclusion there is how you talk with customers. Simply asking people how you can improve your product or being more deliberate and saying "what do you want? If I could add any new feature, what would it be?" is wasted time. These questions feel normal and correct, but they'll lead you down the wrong path. What you instead want to figure out is the contextual situation which has lead your current customer or prospect to you. Figure out what that customer wants out of that product _for them_. That's how you start to build great products.