the used icons must be easily distinguishable between them. To ensure that, vary the icons shapes as much as possible because they are primary way used by the human visual system to distinguish between them before the colors or the representation itself.
the used icons must be exclusively front view or top view representations. Use 3D isometric view only when what you want to represent is precisely matched by a GNUstep standard icon.
Example : home folder icon which is designed in a 3D isometric view can be used in the toolbar to represent the home folder itself.
Warning : don't use the home folder icon to represent the home address in an address book application, create an icon with a new look viewed from the front for this use.
the used icons must use standard GNUstep icons when it is possible/adequate. The IconKit provides standard GNUstep stock icons which have been created to be commonly used accross GNUstep applications. As such, don't create icon variations of the standard GNUstep icons to use in your application, except for the composited icons like the document and plugin icons.
Toolbar icons aren't on the border on the screen then doesn't provide infinite target (see Fitt's laws), then they use a 32*32 size larger than 16*16. Take note that you have the possibility to switch to reduced size 24*24 or to a text representation but that isn't the default.