Sally's sense of self - awareness is crucial. Despite being a creation, she has her own thoughts and feelings. She doesn't blindly follow the norms of Halloween Town. When it comes to Jack's Christmas adventure, she knows that it's not right and she tries to make a difference. This self - awareness makes her stand out and is a very important aspect of her character in the story.
I think the most important thing about Sally is her wisdom. In a world full of chaos like Halloween Town, she is the one who can see the potential problems in Jack's plan to take over Christmas.
Her ability to love unconditionally. Sally loves Jack even when he's making really bad decisions like trying to be Santa. She stays by his side and tries to help him, which shows her loyalty and her capacity for love in a rather spooky world.