What's the number one reason projects fail? What's the biggest title killer in the industry? Scope -- or more accurately, a lack thereof.
What is Scope? The scope of a project is its breadth and depth. What are you making, and how complex does it have to be to achieve what you want?
Scope is something to be decided, not something to be discovered post-facto. This may seem silly, but many projects roll forward without someone taking a step back and asking, "What does doing this really entail?" Scope, most simply put, is the understanding of how vision meets execution.