The first question that would seem to apply when regarding any sequel is "Does this feel like it is of a piece with the first film?" It doesn't have to be the same movie to be a successful sequel, but it should do something interesting. It should either be a response to the first film or a deliberately different type of film or it should build on some interesting story thread or it should enhance our understanding of the world or the characters. By that standard, "300: Rise Of An Empire" is a worthy sequel to "300," stylistically consistent and equally loony, featuring what may well be the first truly can't-miss performance in a film this year.
It would not shock me if, twenty years from now, people talk about this film the way they talk about "Poltergeist" now, simply accepting it as common knowledge that Zack Snyder "really" directed the film. It is so precise in the way it builds off the first film's visual style and so carefully built to wrap around the events of the first film narratively that it feels more like deleted scenes from the first film instead of something that stands alone. That may sound like an insult, but it's not. I would assume Snyder, who co-wrote the script with Kurt Johnstad, probably signed off on every single storyboard, and I am sure Noam Murro was given full access to all the resources that Snyder had at his disposal. It's remarkable how much this feels like it is simply more of the same story, told the same way.