Update February 21, 2019:
This article was originally written in 2013 and I have learned an lot since then. The core idea still holds; if you roast the exact same coffee to two different degrees of darkness, the lighter roast will generally be sweeter. However, there are certainly many cases where a darker roast is superior to a lighter roast and I didn’t make that point clear enough in my original article (partly because I was stuck in the ‘lighter is better’ dogma at the time).
The key point I want you to take from this update and before reading the original article is that lighter roasts aren’t ipso facto superior to darker roasts. However, they are generally sweeter.
I focus on light roasts because the light roast brings out the specific flavors and aromas of the individual coffee varietal, whereas darker roasts tend to make all coffees taste similar; charred, smokey, and ‘roasty.’
There’s also a very specific chemical reaction that occurs about 1/3rd of the way into the roasting process and that reaction is called the Maillard reaction (pronounced “Mayard”). During the Maillard reaction, sugars, amino acids, and water are converted into other compounds and are therefore not available later in the roast. Because I want a sweeter coffee at the end of the roast, I want to leave just the right amount of sugar and water for later roast phases. Therefore I normally scoot through the Maillard reaction as quickly as I can.
See also: The Maillard Reaction: A Practical Guide
The final phase of the roast, called the roast development phase, caramelizes sugars and if you’ve ever tasted caramelized sugar — like what’s on the top of a creme brûlée — you’ll know it is bitter-sweet and carbony. That same flavor and aroma is developed in the final phase of the roast and for that reason, I don’t want to spend too much time in the roast development phase either.
There’s nothing wrong with darker roasts. In fact espresso beans are normally a darker roast but for a sweet and complex cup of brewed coffee, nothing compares to a good, light roast.