I would say good H-scene is something that fits in the ongoing story and is meaningful for character development.
For the contents it's hard to say, different people like different stuff, but I feel like there should be something more than just pandering to certain preferences.
Then again I don't use H-scenes to satisfy myself, so my interest is mostly in the story part. I wouldn't say H-scene is good because it fits my preferences, if it feels out of place I don't usually like it.
Still many of the most enjoyable H-scenes I've read comes from Sengoku Rance, so I'm not so sure myself.
Anyways, what I'm trying to say is enjoyable H-scene is not always good H-scene and good H-scene doesn't have to be enjoyable, if it fits the story.