What defines a Planet?

From its discovery in 1930 up until 2006, Pluto was considered the ninth planet in the Solar System. After the International Astronomical Union revised and solidified the definition of a planet, poor Pluto was demoted to "dwarf planet," with the discovery of similar icy bodies beyond Neptune. A similar reclassification occurred for Ceres in the 19th Century.

Do you think Pluto should still be designated a planet, or not? What about the other four classified dwarf planets? Consider the IAU definition and the characteristics of both planets and small bodies in the Solar System, and defend your position.