My understanding was Pluto could not be a "planet" because it was not a part of the formation of the original solar system due to it being located beyond the frost line boundary [1] which separates terrestrial planets from gas giants.

Basically, anything within the frost line boundary is close enough to the protosun that things like solid water, CO2, et al. melt, allowing for accretion of solid matter and eventually rocky (terrestrial planets). Anything beyond the frost line, and these solid materials freeze up then accrete, and then collect gas from the protosolar accretion disk, forming gas giants.

It's entirely possible Pluto could've been a part of the original solar system but somehow wasn't massive enough to collect gas, or it was a captured object not present during the formation of the solar system.

Either way, I'm sure New Horizons will discover more.

In my opinion, the IAU was correct in downgrading Pluto to a dwarf planet. Scientific observation and study should not be dictated by social tradition.

[1] http://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29