Final answer:
Cilantro is the ingredient in classic nachos that some people might dislike due to a genetic trait that can make it taste soapy to them.
Step-by-step explanation:
The ingredient in classic nachos that some people might dislike is cilantro. While cilantro is commonly used in various dishes for its fresh and citrusy flavor, some people find its taste soapy. This aversion is often due to a genetic trait that makes these individuals perceive the flavor of cilantro differently. On the other hand, ingredients like guacamole, jalapeños, and refried beans are also present in classic nachos, but cilantro is notable for being a polarizing ingredient among many people.