First of all, Emma Watson is Belle in real life. She is Belle. I cannot get over how perfect this casting was.
She collaborated with the director and put a lot of thought into Belle and her backstory. She made sure that Belle used her intelligence productively, inventing a machine so that girls could learn how to read instead of doing chores. She never changes herself based on the opinions of people who clearly don't know her at all. She doesn't agree with her father's way of life, choosing safety instead of discovery, but she stays with him and takes care of him. She doesn't care what she wears or what she looks like, just that she stays true to herself and doing some good in the world.

And of course, her yellow ballgown was stunning. It wasn't a corset and petticoat kind of dress, which fitted Belle's personality very well. All of her costumes were practical rather than stylish, which stayed true to her character. And even though her ballgown wasn't that giant sparkling skirt, it was beautiful.
Belle bravely takes her father's place as the Beast's prisoner. She plans to escape and refuses to adhere to his demands. But then he saves her life, and she saves him in return. While nursing him back to health, they learn to talk to each other. She gets to know the castle, the servants, and a little bit about the Beast's past. She reveals bits of her past as well, particularly about her mother.
Then comes the library.

Unlike the rest of the town, however, Belle is smart. She is concerned about education, kindness, and discovering new things. She tries to improve the town and standard of living, unlike her neighbors who just destroy anything different. The townsfolk are the direct opposite of Belle.
But the Beast is not.
The Beast enjoys reading. His library is massive and beautiful, instantly making Belle speechless. It's not just that the Beast shares this enormous library with Belle, it's the fact that he clearly cares about the same things as her. Their mutual love of reading opens her mind to the possibility that they have something in common, so they could have other things in common as well.

Instead, his goofy side kick Lefou gets the extra character development. Lefou is supposed to be the comic relief, not a funny guy who is sometimes also sincere and serious. I understand that Disney was trying to broaden their audience, and I am not offended at all by the fact that they payed more attention to sexuality in this film.
But the fact of the matter is, Lefou detracted attention. He stole attention from Gaston, he distracted the audience from Belle and the Beast. I loved Josh Gad as Lefou, that was a perfect choice. The musical number, Gaston, was entertaining and fun. That should have been Lefou's most significant moment. There could have been more humor in Gaston and Lefou's antics, but those scenes just fell a little flat.

Props to Emma Watson for taking Belle and making her such an incredible woman. Belle is smart and kind and fearless. Taking on that character, especially when that character is a Disney princess, is no easy task.
Belle and her Beast show us that people can change. Belle never expected to stumble into an enchanted castle so close to her poor provincial town. She never expected to meet a beast, much less fall in love with one. Emma Watson mentioned that she likes to think Belle opened the castle library to the public and used it as a school. I really like that idea.
Belle is still spreading knowledge, fighting prejudice, and reading to her heart's content.