The dark roasted beans start to lose their origin distinction. A dark roast coffee will have flavor notes that rely more on the roasting itself than the unique character of the bean. It will also lose brightness, and gain bitterness, along with a fuller body.
However, you don’t always have to look to a dark roast for great body and fullness. For example, a roaster who knows the craft well can get a great, full-bodied coffee from a good Sumatra coffee without having to roast too dark. Another note about dark roast coffee: it’s a myth that a dark-roasted coffee has a lot of caffeine. The truth is, along with a decrease in brightness, caffeine levels drop as roasts get darker.