... here's 19 reasons why the IAU's Pluto decision was ridiculous. But first, the definition
The IAU...resolves that planets and other bodies in the Solar System be defined into three distinct categories in the following way:
(1) A planet  is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects  orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
1. Nomenclature: An "adjective-noun" should always be considered a subset of "noun". A "dwarf planet" should be no less seen as a type of planet than a "dwarf star" is seen as a type of star.
2. Erroneous foundation: Current research suggests that individual planets do not necessarily cleared their own neighborhoods, and their neighborhoods may not always have where they are. Jupiter, and Saturn to a lesser extent, have cleared most neighborhoods.
3. Comparative inconsistency: Earth is far more like Ceres and Pluto than it is like Jupiter, yet these very dissimilar groups - gas giants and terrestrial planets - are lumped together as "planets" while dwarfs are excluded.
4. Poor choice of dividing line: While defining objects inherently requires drawing lines between groups, the chosen line has been poorly selected. Achieving a rough hydrostatic equilibrium is a very meaningful dividing line - it means differentiation, mineralization processes, alteration of primordial materials, and so forth. It's also often associated with internal heat and, increasingly as we're realizing, a common association with subsurface fluids. In short, a body in a category of "not having achieved hydrostatic equilibrium" describes a body which one would study to learn about the origins of our solar system, while a body in a category of "having achieved hydrostatic equilibrium" describes a body one would study, for example, to learn more about tectonics, geochemistry, (potentially) biology, etc. By contrast, a dividing line of "clearing its neighborhood" - which doesn't even meet standard #2 - says little about the body itself.
5. Mutability: What an object is declared at can be altered without any of the properties of the object changing simply by its "neighborhood" changing in any of countless ways.
6. Situational inconsistency: An exact copy of Earth (what the vast majority of people would consider the prototype for what a planet should be), identical down to all of the life on its surface, would not be considered a planet if orbiting in the habitable zone of a significantly larger star (harder to clear zone), or a young star (insufficient time to clear), a star without a Jupiter equivalent (no assistance in clearing), or so forth.
7. Ambiguous definition: There is still no consensus on what defines having "cleared the neighborhood" - in particular, what the "neighborhood" is.
8. Lack of terminology: Exoplanets - indeed, including any potential Earthlike planets - are arbitrarily declared to not be planets. This deprives those studying exoplanets of an IAU-acceptable term to refer to them by.
9. Inability to describe exoplanets even if not ruled out: There is no way that even if exoplanets hadn't been arbitrarily ruled out that one could ascertain whether a body has met a "cleared the neighborhood" via observations from Earth.
10. Failure to address binary objects. Self-explanatory.
11. Unscientific motivation: The primary reason cited by everyone interviewed thusfar for choosing an exclusive standard over an inclusive standard is along the lines of, "It would be too hard for schoolchildren to memorize the names of all of them". This is such a blatently unscientific standard that it doesn't even bear going into, and leads to absurd consequences when applied to other fields, such as the AMA declaring that there's only 8 bones in the human body and all others are "dwarf bones" that aren't real bones, or the USGS declaring that there's only 8 rivers in the world and all others are "dwarf rivers" that aren't real rivers, all for the purpose of making things easier for students to memorize.
12. Resistance to accept the diversity of reality: In every scientific field, the universe continually presents those making discoveries with a wide range of diversity. This is almost universally accepted in an inclusive manner, subdividing groups into subgroups, and subdividing those further. We will continue to find new types of planetary bodies in a wide range of diversity - large terrestrial planets, dwarf-scale planets, gas giants, ice giants, hot jupiters, super-earths, water worlds, supercomets, extremely large bodies orbiting as moons, planets without parent stars, and so forth. Rather than trying to hide diversity, science is supposed to embrace it.
13. Discouragement of exploration among the public: The term "planet" has a deep and meaningful place in the public mind, as a body worthy of exploration, perhaps even eventually colonization. "Small solar system body" does not. Public support for scientific exploration to these diverse and fascinating worlds should not be discouraged by poorly chosen names. Quite to the contrary, it would be worthwhile if fascinating worlds the diameter of Mercury like Ganymede and Titan were given the same level of attention with a label such as "planetary moons" (note again: an "adjective-noun" is a subcategory of "noun").
14. Distrust of the scientific population among the public: Images of discontent scientists sniping at each other and divisive voting on controversial "truths" have a profoundly negative consequence on the public's view of the scientific community. Anyone who spends any time looking at any of the internet commentary on the dwarf planet decision will find them full of comments along the lines of "Scientists can't even agree about whether Pluto is a planet, why should we trust them about global warming?" I wish this were hyperbole, but I've seen it far too often to ignore it.
15. Poor voting statistical representation: While 4% of the IAU would make up a statistically significant sample if chosen at random, the people involved were not "chosen at random". The people present were "those who could take a trip to Prague and didn't have to leave before the closing ceremony", which leads to numerous potential biases. As Owen Gingerich noted, "There were 2,700 astronomers in Prague during that 10-day period. But only 10% of them voted this afternoon. Those who disagreed and were determined to block the other resolution showed up in larger numbers than those who felt 'oh well, this is just one of those things the IAU is working on'." In this day in age where electronic balloting is simple to implement, that the IAU would be willing to make charged decisions on a 60% vote of a non-random 4% of the membership is highly inappropriate.
16. Wrong people making the decision: Only a small percentage of the IAU are planetary scientists, who are the actual people who should be the ones making decisions about what makes up a planet. Letting people who study stars decide what counts as a planet is akin to letting dermatologists decide how to treat a heart condition - hey, a doctor's a doctor, right? Just like when meteorologists or chemists make claims that global warming isn't real - a scientist is a scientist, right?
17. Making the decision before gathering the data: For most of the history of humankind's knowledge of Ceres and Pluto, we have not had any missions underway to explore them. They were just poorly resolved points of light. But at the time of the IAU vote, at long last, we had launched New Horizons to Pluto and were preparing Dawn for launch to Ceres. Yet it was at this narrow interval, between actually launching craft to gather data about the bodies, but not having them arrive, that the IAU decided to make their declaration. Making scientific declarations about objects that you know little about when vast amounts of data are coming in the pipeline - data that could influence members making the decision - is profoundly unscientific.
18. Not following through on its own declarations: The IAU decision declared that it would continue to name new dwarf planets as new data comes in. Yet there's not been a new declaration since 2006. We have far better data than we had to make declarations of dwarf planets in 2006, and there's a long list of them awaiting declaration - where's the IAU? For example, Quaoar's diameter is known is known to a mere ±5 km and is significantly larger than Ceres. Even the lower bound of 2007 OR10 is larger than Quaoar. Why aren't they and countless others on the list? It increasingly looks like the IAU just wanted to make its declaration purely for demotion purposes rather than for its stated purpose of categorization.
19. Disagreement with the IAU is so intense that those who disagree are simply ignoring it - a process that began in the literature almost immediately (example: http://arxiv.org/abs/0712.2198), let alone in conversations with the public (example: any press conference with the New Horizons team). This not only renders the definition meaningless but serves to undercut the IAU's authority in other issues (such as naming).