In a "pure abuse of power" scene, you have control over the victim's body. But in a punishment scene, you have power over their mind as well. You have power over their very sense of self. You don't just have the power to make their body suffer -- you have the power to make their conscience suffer, too. You have the power to make them feel, not just helpless and frightened and hurting, but ashamed...
Because it would make you feel that way inside as well as out. It would make you feel helpless and submissive, small and bent to someone else's will... not just on your skin and in your muscles and genitals, but in your heart.
And I suspect this is why it feels so dangerous, as well as so enticing.
I've often thought that the punishment kink (domestic discipline, WIIWD, etc.) is among the most vulnerable of practices within BDSM, and I love how Christina articulates why that is. It involves a sort of double jeopardy that is lacking in role play, and the potential for danger can make punishment intoxicating for the participants. The penitent's whole being is vulnerable, his or her "very sense of self" as Christina puts it.
Indeed I think that buzz I've mentioned a few times in relation to punishment spanking is really a sort of high that follows surviving a particularly dangerous act -- hence it only occurs with punishment and not with role play or erotic spanking. Two people plunging into the abyss of shame, guilt, and physical pain and finding their way out the other side still intact -- or, rather, even better and closer for the trip through momentary darkness.