Mathematica handles many different kinds of concepts:
mathematical formulas, lists, and graphics, to name just a few. Although
they often look very different, Mathematica represents all of these
in one uniform way--as expressions.
This uniform expression model is a crucial unifying principle in
Mathematica. The fact that every object in Mathematica
has the same underlying structure makes it possible
for Mathematica to cover so many areas with a comparatively
small number of basic operations, and for each function (whether
built in or user defined) to be so powerful.
For example, there's no need for a special graphics manipulation
language in Mathematica: the uniform expression model means
that like every other kind of expression, graphics can be manipulated
by standard expression-manipulation functions. In a
non-Mathematica system, graphics would have a different
representation type from numerical quantities, so without special
adaptation would need their own set of operations for manipulation.
Diverse concepts are all represented with the uniform expression model.