An icon is more than just a pretty picture. It has many layers that make it up (and I’m not talking about Photoshop layers), and a great icon makes good use of these layers to communicate ideas. Wether we’re talking about tiny 30x30 symbols or detailed 1024x1024 illustrations, icon design follows the same thought process.
Does It Look Nice?
At the most superficial level, an icon should look good. While this may seem highly subjective, there is actually a bit of fashion involved. Like the clothes we wear, icons have styles that gin in and out of fashion over the years. A decade or so the fashion trended towards more detailed and three dimensional illustrations, while today icons skew more towards simple and minimalist.
No matter what the fashion is though, great icons always have a level of polish. Pixel-perfect artwork is immensely more pleasing to the eye because you’re going to get crisp and clean lines. Taking the time to give your icon a pleasing selection of colors is also vital for aesthetics and contrast.
Does it Communicate?
Maybe the most obvious piece, but also somehow overlooked. A great icon communicates its intent clearly, and tells the user what it means without the need for text. Can the user infer the function the icon represents just by looking at it? Remember that something that makes sense to you may not make sense to somebody else, so it’s always a good idea to get varied feedback.
There will never be an icon that will make sense to 100% of people due to things like personal experience and cultural differences. What matters is that you try to create iconography that makes sense to as many people as possible. Not easy by any means, but it’s important.
Is it Simple?
It doesn’t matter if you have pixel perfect artwork that says exactly what the icon represents. If the icon is too busy, then it’s not going to do its job. An icon should be focused on one single idea. Imagine you’re designing an icon for a music app with playlist functionality and a store. Does the icon have all that functionality represented within it? No!
Designing icons for a single function is easy to keep simple. But when your icon represents the software as a whole, it’s important to be able to visually summarize. Use the one idea that best represents what the app does.