Statement: The responses elicited are those as expected from Harry Potter fans--both unintelligent and demonstrating the inability to see beyond the figurative ends of their own noses.
Statement: There are two components to the Cruciatus curse: the incantation itself and the will of the caster.
Evidence: In the battle of the Department of Mysteries, Harry attempts to use the Cruciatus curse on Bellatrix Lestrange. It is ineffective, and she taunts him saying, "You've got to mean them, Potter! Righteous anger won't hurt me for long."
She also explains that the Cruciatus curse is only effective when the caster is causing pain for the (perverse) enjoyment of causing another to be in pain.
Corollary: For the Cruciatus curse to work, it indicates a mentality in the caster: sadism.
Conclusion: The Cruciatus curse is unforgivable not only because it is a curse designed specifically to torture, but the curse's effectiveness is based upon the mindset of the caster that, when effective, indicates that the caster is dangerous to everyone around him or her.