Alan Moore's curse has always been that his stories get adapted by people who don't get them. LXG is a famous disaster, and as far as I can tell From Hell is more inspired by than based on the graphic novel. V For Vendetta is a much closer adaptation -- deeply faithful in many respects and not far off in most. And yet, it changes the message of the original in an unfortunate way, and that change is driven by the changes to the most cryptic character in both stories: the Leader.
It's the single element that the Wachowski Brothers changed most strikingly -- enough so that it has to be deliberate. Let's describe what we get in both stories. Let me emphasize that this is going to be spoilers for both stories, albeit only of one element of each.
In the graphic novel, the Leader is Adam Susan. He isn't exactly the stereotypical dictator -- neither big nor especially loud, shy and a bit withdrawn. He spends most of his time in front of the Fate computer, which monitors everything going on in the country and advises on what's going to happen next. He is a strong hand when necessary, but he is more threatening than violent. V spends a good deal of time carefully destroying him on a very personal level, and it's one of the creepier sections of the book.
In the movie, he is replaced by High Chancellor Adam Sutler. Note that this is, AFAIK, the only character whose name gets changed in the story. I suspect that's because they wanted to avoid confusions from having a male main character named "Susan", but it's somehow apt, because he's also the character who has changed the most. Sutler is the abusively loud dictator, always berating his underlings. He's actually more cryptic in the movie than in the book, never appearing in the flesh until the end of the story whereas in the book he is a significant character. (Really, what we see of Sutler may be similar to what his underlings see of Susan. But by not seeing the man beneath, our impression of him is vastly different.)
The big difference, though, is motivation, and that affects everything that this story is about. In the graphic novel, Susan is a man who has done what he believes is necessary. The country was sliding into chaos -- indeed, as far as we can tell, the world had been practically ending -- so he stepped in and took power. It is quite clear that he believes that order is the only bulwark against disaster, and he is willing to accept any amount of pain that that order inflicts on his society as necessary. In the end, Susan is killed less because of his direct actions than because of what he stands for: the symbol of this society that is so functional on the surface and so broken beneath.
In the movie, Sutler is a simpler villain, who created the disaster in order to take power. Indeed, the movie is heavily about the discovery of him engineering it. He's a true monster, kept at arm's length because we don't want to see him humanized. His killing is the putting down of a figure who is almost pathetic in his whimpering evil.
That single character change alters the story fundamentally. The graphic novel is about the way that society can eat itself when fear becomes its purpose. In the graphic novel, the bad guys arose because the people were so desperately hungry for security that they threw away their freedom for it. Susan stepped into the breach and acted as the force of security, but you always get the feeling that this was a "someone had to do it" moment in his eyes. He views himself as the instrument of Fate, to a considerable degree. The movie, by constrast, is about how a villain can take power by imposing fear deliberately. Adam Sutler is utterly cynical, whereas Adam Susan was utterly (indeed, frighteningly) sincere.
I'm pretty sure that the Wachovskis made the change quite intentionally, as a commetary on modern American politics, and that's understandable -- it's the easy Michael Moore view, of how the US has been taking over by power-hungry madmen. And that's a worthwhile story to tell. But it's frankly less apt than the original story would have been, of how a society in fear often demands a dictator, begging for their rights to be abrogated for a little false safety. The movie talks about how a few bad man can abuse a country; the book is about how a country can destroy itself through fear.
There's a key scene in both stories, in which V goes on the air and tells the populace that this is, ultimately, all their fault. It's preserved from the original into the movie. And yet, it rings hollower in the movie than in the book -- in the movie, the people are guilty merely of naivete, whereas in the book they are more actively complicit. That change makes the movie easier to accept, and permits a happier and more pat ending. But by making it less unsettling, it fails to tell the story that is mostly going unspoken today. And that's unfortunate, because it's a story that people need to hear...