One of the big criticisms I saw was that they tried to get too much done in too little time, which is fair, even if you account for the fact that they tried to get the scope of a 30+ episode series into a 2 hour movie. It is definitely the type of story they could have told over the span of 2 films, if they had decided to do so.

The introduction of Mia was one of the things I didn't like. (SPOILER)Somehow she was simultaneously a cheerleader as well as the "too cool for school" dark chick that smokes on school property. Didn't buy it. I also didn't like that Light was super gung ho about sharing his NOTEBOOK OF DEATH right away with a girl he had a crush on. But I think they did a good job of picking her up from there.

The ending was definitely a huge factor in winning me over.

ending spoilersI'm so glad it was up to interpretation. In a way, it lets you have the ending you want to have. Did L write Light's name down after being heartbroken by the loss of Watari? Did he deduce that Mia was the actual mastermind and figure the right person already had justice served? Did Light end up writing his dad's name down in the book, and they omitted that from the montage of the big reveal?