Deep roasting also does more. Even coffee you buy in a can are roasted to an extent. Even a machine can roast a coffee bean from green to brown. But a roasting expert deep roasts his product, bringing out the utmost body, flavor, and acidity. This gives the finished product a certain viscosity, as well as a lively, energetic feeling on your tongue. All of this can be lost when a bean is roasted half-heartedly.
The catch to full roasting used by experts is roasting the coffee slowly instead of rushing it. It may sound hokie, but bean roasting experts almost -talk- to their beans during the process, to see what they exactly need to be roasted to their fullest potential. It always means, nonetheless, that bean experts thoroughly roast their product all the way through.
If you want a good test to see if beans are properly roasted in this manner, crack one open before you buy it. A good bean, more often than not, will be darker on the inside than it is on the outside. Poorly roasted beans, which have been processed too quickly, will be darker on the outside. Buy these at your own risk!