George W. Bush (or maybe it was Will Ferrell doing an impression of George W. Bush -- it's hard to tell the difference sometimes) once said, "Presidenting is hard." And while President Bush certainly had some unpleasant experiences in office, he didn't have to deal with a nation trying to tear itself apart in a civil war. And he definitely didn't have to deal with vampires.
But Abraham Lincoln (played by Benjamin Walker, who could easily pass for a young Liam Neeson) did. When he was a boy, Lincoln awoke in the middle of the night to see Jack Barts (played by Marton Csokas) near his mother's bed. Immediately thereafter, she fell ill and died, and Abe was outraged. He didn't act on his anger out of respect for his father, but that reprieve ended when Abe's father died nine years later. Barts wasn't hard to find, but he turned out to be much harder to kill than expected. Jack was only momentarily inconvenienced by a bullet through his eye, and Abe's life was only spared by the last-minute intervention of a stranger (played by Dominic Cooper).
This stranger later identified himself as Henry Sturgess, and he also said that Jack Barts was only one of many vampires who had infiltrated society and who were getting tired of the need to hide from humans. Henry was on a crusade against these vampires and, with the promise of knowledge that could help take down Barts and others like him, enlisted Abe's help. Lincoln's size, strength, and intelligence made him a powerful fighter, and the more encounters he had with the beasts, the more he could see their feelings toward humanity mirrored those of slave owners toward their slaves.
You can't possibly have a title like Abraham Lincoln: Vampire Hunter without expecting the film to be at least a little bit ridiculous, and this movie has that in spades. If you want to see a vampire throw a horse at Abraham Lincoln, or a horse-drawn carriage burst through the side of a house and run into a bunch of vampires, or a rifle that has seven barrels, then you can see all that and more in this film. But it doesn't really feel like it's doing this with a tongue-in-cheek attitude, but rather it's taking it all a little too seriously, and that makes it more painful and less enjoyable.
There are also a lot of problems that don't arise from the absurd nature of the story line. For example, despite all the events taking place in the 1800s, everyone seems to use anachronistic modern dialogue that doesn't reflect the speech patterns of their times. There is a severe overuse of intercutting between fast-motion and slow-motion during fight scenes, and a severe underuse of Mary Elizabeth Winstead (as Mary Todd Lincoln) and Alan Tudyk (as Stephen A. Douglas). The film completely skips over most of the politics in a manner that feels very disjoint and unclear on Abe's motivations for changing career paths from vampire hunter to president.
I read the book about a year ago and really enjoyed it. But even though its author (Seth Grahame-Smith) also wrote the screenplay for the movie, they are extremely different. The film covers only a small fraction of what's in the book, while at the same time making up several new things. Almost all of the changes are for the worse, and although it may well have been very hard to make a good movie from the book, the resulting film doesn't really reflect much of an attempt at doing so.