I thought that in the Swedish movies Mikael Blomqvist was too clean, portrayed as just an all around good guy who got hosed. In the books, and what I expect to see in Fincher's movie, is a Blomqvist whose personality flaws become more shining. Something that Larsson wrote into the books, openly states very early in the book is that Mikael is a bad father, Mikael admits it to himself and laments that he doesn't really know how to be a better father. It's a repeated theme. The books make you not sure of the quality of the man's character to start, and then go through the process of winning you over throughout their course. The Swedish films never did that and IMO that made Mikael a more two-dimensional character. That and dynamics of his whole relationship with Erin, the co-owner of the magazine. That's a bit slimy (I hesitate to use that word)/odd, and skipped completely from the Swedish movies, too.
ETA: And the same sort of treatment was given to Lisbeth's character in the Swedish films, too. That's a much longer discussion and it would likely give too away portions of the story too much. The Lisbeth in the books was much more devious and ruthless in the books than in the Swedish movies. If I say anymore in comparison, it'll give away plot points.