One thing I thought the film did particularly well was the casting of Bella. I was not a fan of the books, and one reason for my dislike was I could not empathize with the main character. I felt like I was reading the diary of an overly angsty teenage girl. Granted, perhaps this is why she resonated with so many teenage readers... but I got really sick of how often she emphasized her own averageness. I realize her character was meant to be just "normal" so she would easily relate to many, but I truly felt, most times, that as a narrator, she was trying too hard to sound laid back. It felt forced.
Kristen Stewart captured this "trying to hard to look like I'm not trying to hard" attitude perfectly in the films, and I honestly think it is because she's actually like this in real life. Her acting seemed very natural because I doubt she had to try very hard to be Bella Swan. All interviews I've seen of Kristen Stewart make me feel like she and Bella Swan shared the same (slightly annoying) brain.