I'm no expert on hwo astronomers waste their time and our tax dollars by defining planets, but from what I understood Pluto was downgraded because of it's size and only it's size.
Pluto has an elliptical orbit, but it is a stable one. Back in the day, any heaveny body rotating around a star was consdiered a planet, as it should be. But before this conference started, they decided that other ridiculous things, like size should determine planetary status. The new 10th planet (Xena) as well as one of neptunes moons and a large asteriod were supposed to be bumped to planetary status based on size alone. This is why pluto got nixed because it is smaller than all of these and the scientists didn't want to include all of these odd-ball objects.
In my humble opinion, that is the biggest pile of b.s. I've ever heard. First off a moon of a planet can't be a planet.. we have a name for that, it's called a moon. You know, an object that orbits a planet. Secondly, Xena is a planet, put it in, end of story. Lastly... that asteroid doesn't exactly have a stable orbit and, well it's an asteroid which is a chunk off of something else. Give it time and enough stuff wil be sucked into it so that it's a planet, but for now it's a planetoid at best, not because of size, but shape. I think we would all agree that a blown up planet isn't a planet anymore, it's the remains of a planet. Well that is exactly what an asteroid is.
Long story short, nothing interesting in space has happened for a while, so these wize-guys got together and did this to get some free publicity for the science and screw with our heads. Remember, pluto has changed status about three times since it's discovery.