Reddit mentions of Hunting the Dark Knight: Twenty-First Century Batman

I think the feel was different because for Begins, it was less of a Christopher Nolan movie and more of a Warner Brothers movie, with Nolan being only one of many having creative input into a film. They had to stick a bit more to formula, and rely on familiar Batman elements - after all, they were relaunching a franchise. After the first film's success, both audiences and the studio began to respect Nolan more as an auteur, and he was given much more free reign in making The Dark Knight. Begins was perceived as a smart film that opened the doors for what kinds of superhero films could follow - it paved the way for the second film to be made. Many speak of how TDK transcended genre and became its own film, and not primarily a Batman film.

Will Brooker, probably the world's foremost expert on Batman, writes about this in his new book Hunting the Dark Knight. It's a great read if you have the time.