Throughout the internet you'll find many discussions on "appropriate" levels of BDSM intensity. There are numerous inclusions of what constitutes consent vs. just pleasure, what is okay to do in reality vs. play fantasy, and the like. The views are often shaped by the background experiences of the individual (especially those who have been active in a formal BDSM community), but it seems regardless of a person's experience level, there are many who will share their opinions whether their bias is based on experience, logic, ethics, or fantasy. The greatest levels of D/s intensity generally come under the greatest scrutiny.
The world history of oppressed and oppressors, the slave trade, etc. give ample firepower to those who wish to condemn or speak out against relationships with a high degree of power exchange. Is this really a fair comparison? I believe when people cannot picture themselves enjoying a certain lifestyle there is a tendency to condemn it. The great and over-riding difference between power exchange and history is consent. The power differential is not based upon wealth, skin color, religion, religious decree, race, gender, class, caste, or birth. A Domme doesn't come into existence like a Queen or Tsarina, she isn't born into an existing oppressive power structure with the ability to abuse power merely through title and standing. Her power structure is created when another agrees to fall below her in the heirarchy of the world they create together. This is a might difference.
A sub isn't dragged into his situation kicking and screaming. While he may kick and scream for different reasons later, at some point he made a choice. While I would like to say he enters "with his eyes wide open," in many cases it's with his eyes half open (yes, this is a reference to men who squint when they masturbate). I have often been told that a Domme that arouses a sub in manipulate him sexually and make him agree with something he doesn't really want is doing something wrong or unethical. A case can be made for both views on a personal level but I tend to feel if he "can't help himself," he is still responsible for his choices. A frat boy pillow talking a girl with low self esteem into bed with sweet sweet lies is considered responsible for his actions. A sexual predator seeking the object of his desires and acting out his twisted fantasies is also considered responsible, even if he did it because he "couldn't help himself."
I find it to be an odd double-standard when men follow their penises and some are seen as helpless victims and others as rational beings pulled along by sexual desire.
I also find it odd when people view a Queen abusing her servant in the same light as a couple who choose to live like a Queen and servant. The notions of power by birth and power by consent are indeed very different subjects.
On a side note, I feel that woman's ability to sexually manipulate a man into certain choices isn't an abuse of power. If men were meant to be able to resist feminine charms, our intelligence wouldn't cut in half when we get an erection.