Question

Which attribute disqualifies the object from being a planet?

Answer 1

It must be an object which independently orbits the Sun (this means moons can't be considered planets, since they orbit planets)

It must have enough mass that its own gravity pulls it into a roughly spheroidal shape

It must be large enough to "dominate" its orbit (i.e. its mass must be much larger than anything else which crosses its orbit)

Answer 2

If it is not able to "dominate" it's orbit.  Also, if it is not an object that independently orbits the sun itself.