What do you think is the most basic mistake outdoor home chefs make? What could one do to make the greatest improvement?

Wow, great question. . . so many possible answers. Here is one from www.realgrilling.com.
Put a Lid on It
For four important reasons, the lid should be closed as much as possible.
1. It keeps the grates hot enough to sear the food.
2. It speeds up the cooking time and prevents the food from drying out.
3. It traps the smokiness that develops when fat and juices vaporize in the grill.
4. It prevents flare-ups by limiting oxygen.