*I'll try to keep this short so you don't spend an hour reading this when you could have used that hour to just freaking watch it.
If you've watched enough hentai, you'd notice that hentai doesn't follow the normal plot development. In nearly every hentai, there is no poetic justice. Simply put, the bad guys raping people keeps on raping people without negative consequences. On the other hand, the good guys trying to prevent it and the victims keep getting raped over by hentai-karma. Otome Chibaku Yuugi, however, seems to refute this bland development. For once in a freaking generation, this anime finally DOES have poetic justice paired up with a plot. Needless to say, this is what made Otome Chibaku Yuugi worth watching and, ultimately, superior to the generic, boneU4timesPerEpLolHaha hentai.
Let's face it, nobody dives into hentai for a story (Or at least I hope no one does...) We're in it to see what kind of weird shit happens, occasionally replaced with romance and actual love (still less than 10% of the scenes ._.). The few hentai that actually does have a plot are usually ridiculously laughable (Oh great, he's collecting the 7 holy di***s) and woefully underdeveloped. Fewer if any, actually base the hentai element on the story. Otome Chibaku Yuugi happens to be one of those few. The main character Kaoru was sexual abused and humiliated by his childhood friend Nanao. Otome is the narration of his revenge against her... one step at a time. Not including the actual hentai aspects, the vengeance and conspiracy plot element alone was enough to keep me watching. Watching the story develop to me was like watching an assassin kill his targets... planned out and in secret, he dispatches them one by one...
Comparing the art of a hentai produced some half a decade ago to the standard of art in 2011 is relatively... uh... stupid. Hentai is for content, not for art. Nevertheless the art aspect is important to a certain extent. For example, you shouldn't waste your precious time wondering why you're watching an alien orgy when the summary promised "secret 5th period at school". Art should just be contented at the point where what you see actually resembles what was promised to you. I'll be brutally honest, Otome Chibaku Yuugi art is decent but certainly not the best animation I've seen in hentai. However, the fact that it avoids including those faceless sex zombies with arm length boners adds extra points in my book any day.
How the hell do I rate sound?... Well there wasn't any creepy laughter or puking sound... oh I get it... let's just leave it at "Otome Chibaku Yuugi had the standard average for sound".
I don't know many hentai characters, but Kaoru has to be one of my favorites. Regardless of whether I'm watching a slice-of-life, an action, a mech, etc or a hentai, I am inclined to relate to the male protagonist. I'm sure some of you understand what I'm trying to say. You feel like you somehow become that person in the anime and hence want the best for him (based on what you believe represents "best"). Going even beyond that, Kaoru's personality and his maturing role in the anime would distinguish him even if he was not in a hentai. In an age where male anime characters only seem to start out cold, strong and silent and end the series weak, submissive and fallen-in-love, Kaoru represents the glorious opposite. His trust betrayed by his childhood friend, he becomes heartless and vengeful. Using manipulation and intellectual planning, he achieves his goal of vengeance. What a perfect example of how some male characters should be.
I sometimes can't bear to watch certain hentais because I feel so sorry for a character or it utterly disgusts me. Otome Chibaku Yuugi, on the other hand, had the exact opposite effect on me. With its unique story and poetic justice for its characters, I was voluntarily compelled to keep watching. It was different and it was good. I feel that it would have been a great anime even if the hentai aspects were removed. This, in my opinion, is the sure-tell sign of a masterpiece in hentai.
*Oh and apologies for length-failing again.