After working in a tiny coffeehouse in Philadelphia for two years and later interning with my boss, I've come to develop a pretty high standard for coffee. I thought about exploring the differences between espresso and coffee from a physics stand point (side note: "espresso" is not pronounced "expresso"). The fundamental difference between coffee and espresso is that coffee is created from the drip method while espresso uses high pressure and hotter water in the brewing process. Coffee has a lighter taste to it because the heated water is poured over the ground, roasted bean and allowed to flow through the filtration without interference. Espresso beans have a few properties that contribute to it's thickness and intensity. There are initially roasted for a longer period of time and at higher heat which gives them a darker hue then their coffee counter parts. Before being loaded into the espresso machine, they are more finely ground. These two aspects allow for the some of the intensity within the flavor of espresso. The other huge aspect of why espresso is so much more intense than coffee is in the design of the espresso machine.
The ground espresso beans are loaded into the portafilter, which is then attached to the group head. Water in the espresso machine is converted into steam, which passes through the pressure chamber, and converts the steam back into water forcing it through the puck (ground beans within the portafilter). The pressure of the water flowing through the puck is eight to ten times that of the atmospheric pressure. Pressure is the force per unit area; the larger force of the water extracts more flavor and particles from the ground beans which contributes to the intensity and thickness of espresso. Without this pressure, the only force acting on the coffee beans is the atmospheric pressure, the force of gravity. The difference in pressure explains the lightness of coffee and the thickness of espresso.
Gryphon Cafe, Philadelphia (with a side of quaint)