There a several differences between a darker roast and a lighter one. I divide the taste in the cup into the flavors in the coffee and the roast flavor(similar to the char taste of BBQ or broiling, producing carbonic acid). The darker the roast the more "roast" flavor will be in the cup. There are also various flavors in the coffee that will be stronger or more subdued depending on the degree of roast. Coffees that have fruited
(berry, stone fruit, apple, lemon, ginger and or nutty flavors (depending on the origin and varietal and type to processing)will shine at a lighter roast while chocolates will stand out more at medium to slightly darker levels. Vienna and French roast levels will be dominated by "roast" tastes. The quality of the coffee beans is much more important in light roasts. The growing conditions, care in picking and sorting, insect damage, molds, care in processing, handling, shipping, storage etc. will all effect the final quality. These are called defects in green coffee beans. Many beans are only dark roasted to cover up the defects in the beans. Light roasting quality beans takes much skill but when done properly yields a fantastic cup. I highly recommend trying lighter roasts from a quality Roaster.
check out this related post: "Fresh" Roasted Coffee Beans