Second in popularity only to actually observing stars, astronomers are once again debating what, exactly, a planet is. Depending on which way the argument goes this time, we might "lose" Pluto or "gain" Ceres, Quaoar, and Varuna.
One of the things that has always struck me about Pluto is how different its orbit is compared to the rest of the planets. All the others orbit in essentially the same plane, with (as I recall) less than 5 degrees difference. Pluto's is inclined very steeply, with its apogee well "above" the other planets.