In the past, I didn't plan things well and left out major work elements. I once built a billing system with microfiche output but forgot to include the microfiche readers. If you've done a reasonably comprehensive plan from your side and from the user side, something like that will stick out. Be aware that managing change is crucial to good project management. Manage expectations so that people understand something may cost more or that you need to shorten functional lists.
People are important, especially the smart ones.
Work hard to find employees with the right background, attitude, hunger and energy. Give tangible evidence that indicates you believe people are an important resource. When building teams, start small. With the right energy a small team can either solve a problem or get more resources, while too-large teams flounder. Long discussions about overarching goals or vision statements won't necessarily give you a lot back.