A really big budget, no external time constraints, and a marketing budget bigger than any of us had ever had before made us soft. For one, there's no way, in a first-person RPG, to stage a raid on a POW camp to free 2,000 captives. Also, there's no way to re-create all of downtown Austin, Texas, with any degree of accuracy. Third, blinded by the power of UnrealScript, many of our original mission concepts depended upon special-case scripting and lots of it. Find your focus early and maintain that focus throughout.
General solutions are better than special casing.
Give players a rich but limited tool set that can be used in a variety of ways, not a bunch of individual, unpredictable solutions to every problem. Always work within the limits of your technology rather than trying to make your technology do things it wasn't meant to do. Big budgets, lots of time, and freedom from creative constraints are seductive traps. Don't fall into them. Don't settle for less than greatness, but don't think too big.