Why do the un-health conscious fight over the skin of a peking duck, a well-roasted chicken, or the burnt ends of a brisket? And why do coffee, chocolate, bread crusts, and dark beers taste so gobbledy-yum? Because dammit, they have complex flavors, thanks to the Maillard Reaction.
The Maillard reaction occurs between 250F and 500F (some say 300F and 500F) and is defined as the reaction between proteins and sugar on the surface of a cooking product (meat, bread, what-have-you) causing the surface to brown and a complex flavor to develop. There's a lot of good science (and not so hard to understand, if I do say so my unscientific self) behind the Maillard reaction, which Harold McGee explains extremely well on page 778 of On Food and Cooking, Second Edition.
The Maillard Reaction is so much more than caramelization, "the simplest browing reaction" (Harold McGee), as it involves amino acids and a chain of reactions that results in complex 'beefy', savory, meaty, chocolately, earthy flavors along with caramel flavors.
Because water temperature doesn't rise much above 212F, boiled foods do not undergo the Maillard Reaction. Nor do steamed foods. Dry methods work best for catalyzing the Maillard reaction - roasting, grilling, frying and baking. Slow cooking above 250F will result in the development of extremely deep, rich flavors, where quick cooking at a much higher temperature will rob a food product the opportunity to develop those flavors.
One exception -- long, slow cooking at temperatures below 250F, alkaline conditions and high concentrations of carbs will trigger the Maillard reaction. Boiling down chicken or beef stock will trigger the reaction (think glace de veau). Cooking egg whites in boiling water over 12 hours will result in deep, dark whites according to McGee (I've never tried this myself...the thought is a little repulsive).
Flavor companies rely on the Maillard reaction for the creation of synthetic savory flavors. Betcha didn't want to know that! And betcha didn't want to know that a lot of ready to use meat products have lots of these synthetic flavors in them. I've been to a class at a food company that discussed this stuff.
I know. I, unfortunately, know.