"What's in a name?" is an important quote. I think icons are really important in the digital world. That why I say: "What's in an icon?"
An app icon isn't just an icon. The users get their first impression of your app with looking at the icon. I think we can say it's one of the most important things. If your icon isn't beautiful, users won't take a further look at your app and you won't sell your app to all these persons who don't like your app icon.
But what is a great app icon? An amazing app icon has got some key elements. These are simplicity, recognizability and uniqueness.
Your app icon has to be simple. Don't use a lot of colors, but focus on just two or three colors. Don't use too complicated designs. Try to summarize your app functionality into your icon. In that way people will instantly know what your app is about. For example the mail-app has got an enveloppe. If users see the mail-icon, they know that it's the e-mail app because of the use of an enveloppe in the design.
Your app icon has to be recognizable. Even if your icon is really small, users will have to recognize it. In the search-section of iOs, you can search all things, even apps, but the icons are really small in that section. Your icon needs some contrast in colors and a simple design in order to recognize it in the search section.
Your app icon has to be unique. Don't copy a design, or make a little change in an existing icon. Your app icon will be associated with your app and you can't really differentiate your app if you use the same design (with some tweaks) from another app. Try to think outside the box.
Finally, don't make the glare yourself, iOs, will do it for you. In Xcode you can simply set use app gloss to yes and your app will have the well known iOs-gloss.
Here you can find more information. This is an official Apple Developer document.
Here you can find more tips concerning icon design.
What is you favourite app icon design? Why do you like it?