Flavor is a general measure of the overall taste of the espresso. While the other review criteria certainly influence the flavor rating, I paid close attention to an espresso's depth of flavor, its complexity, its aftertaste, and my general appreciation.
I gave higher ratings to an espresso that exhibited greater flavor depth (is its flavor robust or subtle?) and complexity (does it express a variety of tastes, or is it one-dimensional?). Other factors that influenced an espresso's flavor rating included its freshness, whether it tasted sour (usually if it's brewed at too low a temperature), and whether it tasted ashy (often from over-roasted beans or if it's brewed at too high a temperature).