Some theoretical stuff to add. What follows is taken directly from "Bosch Fuel Injection & Engine Management" by Charles Probst.
Rich mixtures increase fuel consumption and emissions of hydrocarbons(HC) and carbon monoxide(CO) - the products of incompletely burned gasoline.
Lean mixtures reduce power, elevate engine temperature, and increase emissions of oxides of nitrogen(NOx) - a product of combustion at excessively high temperature. They also tend to cause driveability problems.