The roast degree (how light or dark the coffee is roasted) has the most profound effect on the coffees taste. It is the easiest identifiable coffee characteristic. The second most obvious characteristic I would say would be whether the coffee was dry-processed or wet-processed, but the way the coffee is processed and the degree it is roasted to often go hand-in -hand (most wet-processed coffees are roasted lighter, while dry-processed coffees are roasted darker). A roaster selects a certain roast degree for a coffee to bring out the specific coffee's inherent talent. If a coffee from a specific estate has the ability to have a wonderful acidity and great enzymatic flavors, the roaster would be foolish to roast this coffee dark, because then much of the acidity and enzymatic flavors would be roasted out. As a coffee is roasted
1. acidity peaks very early and decreases throughout the roast
2. body peaks at a medium-dark roast and tapers back down
3. different natural aromas inherent in the beans are emphasized -- in light roasts the enzymatic aromas (flowery, fruity, herby), in medium roasts the sugar browning aromas (nutty, caramelly, chocolaty), and in darker roasts the dry distillation aromas (resinous, spicy, carbony), and
4. past a medium roast, natural coffee sugars and aromas are roasted out in lieu of carbon (ash).
Coffee beans that are wet-processed (as are most central and south Americans, and Africans) lend themselves better to lighter roasts because wet-processing engenders a cleaner and brighter acidity; while dry-processed beans (most Indonesians) lend themselves better to darker roasts because the processing engenders more rich earthy dark tones without acidity. This all emphasizes that most of what one tastes in a cup of coffee are the processes the coffee went through and not specifically its origin; though coffee from a specific origin generally undergoes the same processes all coffee from that origin has for years.