This book is a mystery with some thriller elements. There is nothing wrong with the book, but I do not see why it became a phenomenon. There is a mystery with another mystery book-ending the main mystery. I was not entranced by that, because when we came back to the book-ending mystery I did not really care.
The characterization of the two main characters is fine. The other characters, especially the main villain (and any other villains) are horribly cliche. The writing style is simple, moves fast and easy is to read, but lacks all literary merit.
The mystery itself is fine, and the Girl with the Dragon Tattoo is a pretty good character. The whole thing is much too long.
Its a perfectly fine mystery. I suspect that the movies may be better than the book, because it is more that kind of a thing really.