The difference between perfectly roasted coffee and a ruined batch can be a matter of seconds. Roasting brings out the aroma and flavour that is locked inside the green coffee beans. A green bean has none of the characteristics of a roasted bean. It is soft and spongy to the bite and smells green, almost 'grassy.'
At lighter roasts the bean will exhibit more of the characteristics of the origin of the bean. This is determined by the weather, location, altitude, soil and variety of the bean.
Darker roasts have reduced acidity therefore the 'bitterness' of the bean is more noticeable. However the origin of the bean can be difficult to distinguish. Darker roasts lend themselves perfectly for milky coffees adding more depth to your cup.
A huge amount of research has gone into the commercial roasting of relatively low-quality coffee, most of it to do with the efficiency of the process and the methods used in producing instant soluble coffee. As these coffees aren't particularly flavoursome, very little work has be done on the development of sweetness, or the retention of flavours unique to a particular coffee's terroir or variety.
Speciality roasters around the world are, by and large, self trained and many have learned their trade through trial and error, each company has it's own style or roast philosophy. They may well understand how to replicate what they enjoy drinking but they do not sufficiently understand the whole process to manipulate it to a variety of different style roasts. That is not to say that delicious and well-roasted coffee is not rare - it can be found in most corners of the world. There is a bright future for quality coffee roasting as there is still a lot to explore and develop.