Answer: Tail Orientation
Almost everything about a comet varies from one particular comet to another. Some are tiny and only a few hundred feet across while others are substantially bigger and are tens of miles across. Some are more ice than rocky particles and some are rockier with less ice. Even the mixture of gases in a comet can vary significantly between these celestial bodies.
One particular feature, however, is consistent across all comets. Regardless of their size, speed, or composition, the tail (Ion, type I) of a comet is always oriented away from the star it orbits. Why is this physical trait so consistent? Because the tail itself is a byproduct of solar winds and whenever a comet is close enough to the star within a given solar system, the solar wind is always pushing away from the star and directing the tail away from the solar body.
Image courtesy of the Yerkes Observatory.