Alright, enough with the NASA bashing.

First off, NASA didn't make the decision. NASA focuses on aeronautical engineering and space engineering. The definition of a planet would be categorized as astronomical, thus not in NASA's purview.

NASA is also a national organization, and thus does not have purview to decide for the entirety of humanity what is a planet or not.

The association which took the decision in a meeting in Prague was the International Astronomical Union.

Secondly, the decision was not simply "Alright let's just say Pluto is not a planet". Astronomers found an asteroid (Sedna) which was bigger than Pluto. It conformed to the previous definition of "planet" without actually having the characteristics we would expect to find in a planet.

The goal of having a definition is to make an ensemble that we can make generalizations for. If when we try to make generalizations, but always have to exclude some objects, what good does the definition have? Per this, neither Pluto, nor Sedna fit the generalizations of planets, while still being categorized as such.

So the IAU had a choice : either it could accept an asteroid as a planet, with many of the characteristics of planets applying to "all planets except Pluto and Sedna" or it could redefine "planet" as something for which the generalizations would apply.

The categorization of Pluto as a planet was already a stretch, only being categorized as a planet because of other categories not existing.

With Sedna coming into play, it was evident that a better category would be needed. Thus the category "Dwarf Planet" was created, with Sedna and Pluto being categorized as such.

For another example of term redefinition, this was also done with the classification of 1 as a prime number.

The decision had to be made, but it angered some people who dislike change and educators who would have to change lessons and whatnot.

Nevertheless, it was the right decision. If you guys had real reasons other than "When I grew up Pluto was a planet", discussing them would be interesting, but as this is not the case, please stop the NASA-bashing.

Edit : RiflemanLax made me look again at which semi-planet was responsible for the change. It was not Ceres (as I previously suggested), but 90377 Sedna. (Also, see Edit 2)

From Wikipedia:

The discovery of Sedna resurrected the question of which astronomical objects should be considered planets and which should not. On March 15, 2004, articles on Sedna in the popular press reported that a tenth planet had been discovered. This question was answered under the International Astronomical Union definition of a planet, adopted on August 24, 2006, which mandated that a planet must have cleared the neighborhood around its orbit. Sedna has a Stern–Levison parameter estimated to be much less than 1,[d] and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity. To qualify as a dwarf planet, Sedna must be shown to be in hydrostatic equilibrium. It is bright enough, and therefore large enough, that this is expected to be the case.[57]

EDIT 2: Eris was also one of the !planets that were considered in the change. As per RiflemanKax.