In the 10+ years I've been running a UI/UX design firm for software companies, I've come to the conclusion that icon design, is not very different from logo design and brand design. A Nike swish logo by itself, without context of the brand we have with it today, is just a random piece of graphic. The pixels itself are worth practically nothing. But because when we see the Nike logo, we feel something (Athleticism, motivation, inspiration, energy, etc.) the mark is worth billions. The magic is not in the mark itself, it's in the context and meaning it builds up in the user's mind over time. Same goes for icons. When seeing a play button with a red background and a white triangle, it might suggest you're looking at a Youtube video. Icons, have no real meaning or worth without context that's built up over time. Both are 'investment graphics'. So the purpose of an icon should be to make the user familiar with something and initiate a mental model when you see it (You see Nike, you feel something. You see a Play button, you know you need to click it, etc.). Don't judge icons by their looks, but by their context. In other words: Can the icon you are adding in your UI, over time and by continuously link the mark to a specific outcome, make the product easier to use because it instantly gives the user a mental model of it's meaning and consequence when clicked.