You mean that stuff that enhances the flavour of what you are cooking?

 Charcoal makes more smoke than gas, with a broader range of tasty flavor molecules, because it is burning complex organic molecules, among them cellulose and lignin. Gas is a simple molecule, and when fully combusted, produces no flavor, just water and carbon dioxide.

Additionally

charcoal can get hotter than standard gas grills without infrared burners, and heat is what you need to get steaks and lamb chops crisp on the outside while keeping them red or pink on the inside.

Gas grills win on convenience, turn a knob and it's on. But when grilling I want the meat to taste the best I can. I don't care that gas let's me be lazy. Basically if taste is paramount to you, then go charcoal. If ease of use is paramount, go gas.