According to National Geographic online, planets should be large enough that they dominate their neighborhood. Because Pluto's moon Charon is half as large as Pluto itself, it cannot truly be said to dominate the neighborhood (consider other planets are much much larger than their moons). Also a planet should be large enough that its own gravity has caused it to become round by drawing in asteroids. Lastly it has been brought up that Pluto's orbit around the sun is not orderly enough to be considered a planet.
So those are some of the reasons why Pluto lost its planet status.