Why is Pluto not a planet?
I think the primary reason it was demoted is because it's orbital plane is very different from that of the 8 planets. The other 8 planets have an orbital plane close to the equatorial plane of the Sun. This would suggest it was not part of the Sun's original accretion disk.
Pluto's orbit is also much more eccentric than that of the 8 planets (less like a circle and more elliptical). Even as kid, I didn't feel Pluto belonged in the family of planets, and I was happy to see it reclassified.