In KDE there are basically two types of icons. There exist special guidelines for each of these:

Toolbar icons are very often concrete icons They are pictures or close representations of the operations which they represent. Toolbar icons are generally used much more than application icons. So one needs to find them and recognize the purpose they resemble fast. Therefore they are usually more symbolic and simple than application icons to improve usability. Toolbar icons are tools one just wants to use. Making them toodetailed would decrease usability a lot.

Application icons being used to start applications, to resemble folders, mimetypes and devices. They are are abstract designs that may have only a superficial or simplified representation of the operation. Some bear no relation to the functionality at all. Instead application icons try to be much more unique, original and beautiful. They are the brand of the application and are used to "advertise" the application (Think of the CorelDraw icon - If you wouldn't know what this icon is about you would never guess that a vector-graphic-application starts once you click on it).