This intro to psychology course has taught me a wide range of things. Some of the things I've learned have seemed like common sense, while others have really surprised me to be true. The one thing that I will remember five years from now is what helps me understand how some of the seemingly wrong things are true, the Six Principles of Critical Thinking.
So maybe I cheated a little and picked six things instead of one, but I think the principles are always something I'll have in the back of my mind when analyzing something new. The reason I'll remember these principles for so long is probably because they've been pounded in my head all throughout the course. I think they've been repeated so many times for good reason, though, for they really help to overcome possible biases when deciding what you think about a new subject. I've frequently found myself referring back to Occam's Razor when debating with people, and I am now much more skeptical towards things with extraordinary claims without extraordinary evidence. Basically, I think the principles are important because they can be put to use in almost any situation in life. Five years from now, I might not be able to impress a date by explaining the processes of each part of the brain, but at least I will be able to properly evaluate everything she, or anyone else, tells me.