It's so hard to say what Ouran is, it might just be easier to say what it isn't. Ouran is not serious. Ouran is not supposed to make sense. Ouran is not supposed to be remotely realistic. It will not change your world view. It does not have a deep, philosophical meaning, and is not supposed to make the reader want to go out into the world and do great things.

And that's what makes it great.

It has an overused cast description (and makes fun of the stereotypicalness itself). You have the prince, the glasses character, the twincest, the lolita, and the stoic type - something for everyone! The setting is completely unrealistic, the activities outrageous, the behaviors creepy and crazy. But the beauty of Ouran is, that's exactly the point! The entire story is based on the idea of all these characters and situations being completely absurd, hilarious, and yet still strangely relatable for the reader.