For me, it is a tie between the Caspian-Susan romance, Peter's arrogance, and Susan fighting like Legolas. The first one takes away from Caspian's awe of the Pevensies. He can appreciate Susan's beauty, but because she is a myth becoming true, I don't think he should view her romantically.
As for Peter's arrogance, I disliked it because it made the High King, the highest authority in Narnia (besides Aslan, of course), a selfish jerk. He eventually humbled himself, which I am glad about, but his attitude was not becoming for a mature king of Narnia.
Lastly, I was unhappy with the way they had Susan fight, since it was totally out of character. It is one thing for her to fight, as she would want to protect Narnia, but it is another thing to make her a warrior. She is Queen Susan the Gentle; she does not like to kill things, as glimpsed with the bear scene and expressed in the book.
Well, that is my opinion. Lol, it is quite long for my first post on this forum.