1. From a marketing perspective, it incorporates your branding: It has to be easy to decipher, easy to recognize, and it should employ the main strengths of your brand.
   From a usability perspective, it needs to give the users hints about your apps main functionality. But it is strongly advised not to overstimulate this: If the actual app fails to fulfill the expectations that the icon sets, you will burn your users.
2. From a graphic design perspective, the icon has to look beautiful, strong, and not out of place when it is composed into its actual environment (i.e. the iPod Screen, or the Mac OS X Dock).
3. From a semiotic perspective, the signs and symbols that you use should have a shared and distinct meaning in your audience. If you intend to use symbols (like a note, a truck, a cup, a newspaper, or a book) you should make sure that they fit the attribution you are trying to apply in general.
4. Finally, the icon is the entry point into your app experience. And in most App Stores (iPod / iPhone, Ovi, Android …) the icon also represents your advertising in the store. It is the first thing people see, and the icon quality can influence the decision whether people like to request more information or not.

Now, that does not mean that only perfect icons will lead to success (as Google has shown time and time again), but if you have limited other advertising resources at hand, then it is certainly advisable to optimize the hell out of your icon.

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