Gerrymandering the definition
Unlike Mark, while I do not think Pluto 'should be a planet' or even should not be a planet, I 100% agree with him and others here that the issue is a workable definition about what is a planet. It certainly seems that the IAU subcommittee started with their preconceived notion and subsequently gerrymandered the rules to fit. In that, they did a terrible job, e.g. with their clearing it's zone ill-defined requirement that is so ambiguous that planets like Jupiter, Neptune and Earth with Trojans, resonant co-orbitals etc. are in question.
Given that the non-astronomical public would react, it was a really lousy job by not providing a defensible definition that leads to a better understanding or description.