I have a old propane BBQ with a lid that holds in the heat and volcanic rocks that get hot. I've been to two parties lately with new BBQ's, no rocks and no lid. They don't get near as hot and take much longer to cook. And they have to use a lot more propane.
If you wanna cookout you better think about propane Cause your friends have come and the food ain't done propane You can't broil, you can't boil, you can't fry Propane
A cover allows your food to bake, rather than grill. Good for chicken and corn, which need more time and that type of environment. I've always done steak, burgers and hot dogs on an open grill.
Lava rocks, ceramic briquettes and other such stuff is about evening heat, where many burners will have heavy differences between hotter and cooler areas. They also hold the heat, which vaporizes drippings, providing for smoke flavor and a more "charcoal" taste. Multiple burners, properly designed spaced, provide more even heat, and flavorizer bars (or similar metal grease management) will similarly provide the smoke effect.
A good, well designed gas grill should allow you to cook in multiple ways. It also works faster than charcoal (this from a charcoal user).
