I don't really like the IAU definition being taken as law, considering... what authority does it actually have in anything? The vote was sketchy too, for sevearl reasons. Why do non-planetary scientists get a vote? Why was the vote held when there were fewer people at the event?
The "official" reason I've been told is that there was a push for something called the "geophysical definition" which basically says "if something is round due to gravity, but it does not undergo fusion, then it's a planet." This "upset scientists" because it would mean the moon is a planet. "There would be too many planet!" I don't see this complaint with stars, or asteroids... But okay, restrict to so that "if it orbits another gravitationally rounded body that is not undergoing nuclear fusion (caveat, caveat, caveat, and etc. for the particularly pedantic), then it's a moon. Personally, I don't see why something can't be a planet and a moon.
Now that is a fairly obvious solution and definition, however it would still mean we would gain a lot of planets, most of which were discovered by one American, Mike Brown. The rumor is that this upset a lot of the Europeans at the event (Europeans discovered Uranus, Neptune, and now Ceres, while Americans discovered Pluto, Sedna, Makemake, Eris, Haumea, and Quaoar), and so hence the really stupid definition they came up with (1. round, 2. orbits the sun, 3. if we feel like it really earned it).
What's funny is that planetary scientists still call things like Pluto a planet in their papers, usually after a preamble talking about Dwarf Planets in the introduction.