The subtleties of light were one important part of it. They tried to focus on the overall effect, trying to portray an impression of movement through time and provide a sense of immediacy, rather than focusing on small details like lines and contours. It's less photo-realistic than most artists were aiming for at the time but it tries to engage the viewer in ways other than a pure visual reference. They're also typically outdoors scenes - previous almost all painting was done inside a studio, but the impressionists moved outside, where the light was less uniform and predictable.