Icons should be recognizable at all levels.

The Symbolic level. A notebook and pencil together are
symbolic for a simple text editor, a notebook and a fountain pen for a
more advanced text editor. A car, a bucket and a brush symbolize an
application to wash a car.

The Object level. Objects often have an
archetypal form and deviations from that form. You can have a classical
watch or a Swatch. Someone looking for a tool to set the system time
may overlook the Swatch, he will not overlook the classical watch.

Each one of us has an other image of a classical watch in his mind. You
may have experienced being in a supermarket, looking for a certain
product, while having the wrong image of it in your mind. It is hard to
find then, while with the right image in mind it is spotted instantly.
For this reason the artist should not follow his own image and paint it
as he sees it in his mind's eye. He should bring it down to the
essential elements. These are often cartoonish. Instantly recognizable
for all.

It is time to mention the general design principle KISS - the
abbreviation stands for "Keep It Simple, Stupid". KISS applies to all
fields of design, whether it be cell-phones, taxes or icons. While
artists may love to paint beautiful paintings, icons need simple
symbols. Think Icon, Not Painting. Be a follower of KISS.

The Shapes
level. You travel by train to a big city you do not know yet. You pass
the industrial outskirts, you see many non-descript buildings, and
forget them immediately. On the other hand, if you ever saw an image of
the Empire State Building, you will always recognize it. Some shapes
stand out, others do not. You will not mistake a Mondrian for a Monet.

The Assembly level. Shapes together form an assembled shape. This shape
too can stand out or not. Icon sets may enhance the assembly shape with
an outline. An example of an assembly shape is the skyline of New York.

Here we can mention the empty shape too. You probable know the images
with two faces looking at each other, seen en face. Some see the two
faces, other see a chandelier in between them. With two wine bottles,
one a bit in front of the other, in between them you can see a
beautiful v like form. The empty shape is the glue in images. It too
can stand out or not.

The Sizes level. Now we scale the image down to
16x16. All details are lost, but that should not be a problem. Do we
still recognize the notebook and the pencil? Can we distinguish the
pencil from the fountain pen? Do we exclaim: Hey, a miniature Empire
State Building? Do we recognize the skyline of New York? Is it, though
smaller, basically the same image?

With this in mind, log out from KDE,
start an old-school window manager, in which the icons were drawn pixel
by pixel. Poke around, look at the bigger and the smaller icons. Which
ones are good, which ones are not? Do these rules apply, can you make
better rules? Then log into KDE again, look at the icons. What do you
see?

Pixels vs. Vector

Icons are pixel images or
vector images. Both have their pitfalls. Understanding these pitfalls
will make you a better icon maker.

Icons used to be made as
pixel images. Often the artist would make a 32x32 icon, and then
scale it down to 16x16. Scaling the icon down makes it fuzzy, the
artist had to repair it. Sometimes the 32x32 icon had to be
simplified in order to make it possible to scale it down. Going back
and forth between the icons the artist produced two icons that looked
the same. Each time a new size is added a new icon has to be made.
The number of icons in a set can become so big, the icon set becomes
unmaintainable. Enter vectors.

Vector images can be
scaled. This way, theoretically, an artist just has to make one icon.
Unfortunately, at small sizes every pixel counts, you can't leave
scaling to small sizes to a program. You need hand-optimized icons
for the smaller sizes anyway. Another problem with vector icons is
that they are often made at the spacious 128x128 size. The artist
gets lost in space, starts to make landscapes full of details. Scaled
down you just get a blur... Artists new at icon-making miss a
historical background, they did not learn in the pixel era to keep it
simple.

Toolbar and Application Icons.

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).