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.