I was quite pleased at the difference some very simple things can make, such as the light boxes/circles in the iris, the drop shadow at the top, and emphasizing the circular nature of the eyeball. I'd previously ignored basic things like depicting the eyelids, and the "beak" shape of the tear duct is a handy callout. I'm most pleased with the anger/fear expressions, as the personality very much shines through and suggests a sense of character and scenario.