The primary goal of an icon is to convey information. The best way to achieve that is to use a limited set of concepts in an icon: simplicity should be your goal. Schematic design is generally a good idea; but if you want to use a photorealistic look, pay attention of not beeing too much realistic or over-realistic: your icons should be easily readable.
Icons should look emotive but not artificially transparent, reflective or metallic. In order to achieve that, keep in mind the two advices below:
use gradients sparingly when you design your icons.
use shadows and transparence sparingly when you are dealing with the finishing touch.
Last point, the icons should also be designed for graceful degradation when they are scaled. To achieve that, be sure to create your icons with an outline (what permits to keep the different objects used in the icon distinct) still visible at size like 16*16 by having more emphasis on the line weight and/or contrast than the patterns, lighting and gradients; thr patterns, lighting and gradients which should gradually blur when the icons are scaled down. When needed, don't hesitate to create specific icons representations for the sizes like 16*16 and 32*32.