I was talking about this with a friend a week or so ago. It's easy to forget that the catalyst for so much of what has gone on was due to Jaime and Cersei being afraid their incest would be revealed.

As for the incest itself, I find it to be a clever device to quickly paint a character as good or bad. You'll notice as the show has tried to condition Jaime and Cersai to be crowd favourites their physical relationship has been less in your face and it's not even communicated that much either. When they wanted Jaime to start becoming more likeable they had him off flirting with crowd favourite Brienne, while Cersai was back at King's Landing smurfing her cousin. They use it as a socially recognisable taboo that we associate with bad, and it works pretty effectively.