Crosses the Line Twice: This is the unintended reaction of many modern viewers. The racism is so extreme that it's ridiculous—and pretty darn funny. The ending, which ends with the KKK intimidating black voters away from the pollsis supposed to be a Happy Ending.
Designated Hero: Ben and the rest of the Ku Klux Klan. Even if you somehow, by some effort, conjured the perspective of the film's intended audience, a bunch of vigilantes who deny franchise to African-Americans must be hard to accept as heroic.
First Installment Wins: The film had a sequel called The Fall of a Nation, which is probably the first movie sequel in history. It did not involve Griffith or any of the original cast, but was rather directed by Thomas Dixon, the author of the original novel. The film is set in the near future and depicts the U.S. being invaded by a German-dominated Europe. Basically, the film was a plea for the U.S. to enter World War I and stop Germany before it's too late. The Fall of a Nation was a commercial failure and there are no known surviving prints. You might be able to track down a copy of the book it was based on.
Alan Moore: The origin of capes and masks as ubiquitous superhero accessories can be deduced from a close viewing of D.W. Griffith's Birth of a Nation
Genre Turning Point: In the words of critic Dave Kehr, "here, in a very real sense, is where the movies began, both as an art and as a business."
The Birth of a Nation was the first blockbuster in film history. Historians have a hard time figuring out how much money it made, but some have argued that adjusted for inflation it made nearly as much money as Gone with the Wind (which did break the record for most earning on a single year). The film's success in America made producers realize that the motion picture business could be profitable and since Griffith (who was an independent film-maker) had made the film in Hollywood, it led many to follow suit. Of course, the film was preceded by Cabiria in terms of an Epic Movie with a feature length runtime, but Griffith's use of the medium to portray a setting familiar to American audiences (and in living memory at the time of release) codified it.
The ground-breaking technical achievements of The Birth of a Nation (cross-cutting, use of close-ups, long-shots and medium shots to separate action and delineate emotional involvement), its battle scenes and location shooting inspired film-makers across the world, of all stripes and political persuasions (including Soviet film-makers, who being anti-racist in theory, needless to say, did not share the film's ideology).
The film's controversial reception, it's involvement in the rise of the Second Ku Klax Klan (who kept renting out the film and routinely screened it in the 20s and 30s), the backlash by the NAACP and the film's use of technique in support of dubious ideology, has made the film (along with The Battleship Potemkin and Triumph of the Will), a perennial point of contention on movies as propaganda and the responsibility of artists to depicting history. To Hollywood's credit, the backlash by the NAACP against the film did make them cautious about subject matter, and few movies made later are quite as explicitly racist (implicit racism is another thing).
Ho Yay: The "chums." Pre-war, they're teenaged boys frolicking and walking around with their arms around each other's waists; on the battlefield, they die in a tender face-to-face embrace.
Memetic Mutation: This movie is generally blamed for making fried chicken racist.
Moment of Awesome: Unintentional, but watch the scene where Silas Lynch enters Stoneman's office where everybody but him is white. Stoneman tells Silas Lynch he doesn't have to bow and that he is the equal of any man in the room.
Never Live It Down: This pretty much permanently tainted D.W. Griffith's career and legacy. His incredible revolutionary work as a technical innovator has been forgotten. Commercially, the film was a huge box-office success and it allowed Griffith to remain an independent film-maker but none of Griffith's later films (barring Way Down East and Orphans of the Storm) were as successful, and the two films he made to prove he wasn't racist, Intolerance and Broken Blossoms were unsuccessful.
Seinfeld Is Unfunny: All of its innovations have been a standard component of film grammar for one hundred years. About the only thing that is left that makes the film interesting is the content, which is shocking for its open racism and internalization of the discredited Dunning School and Lost Cause historiography.
True Art Is Ancient: Subverted. You'll be struggling hard to find someone who praises the movie without adding a lengthy disclaimer that they don't agree with its message. Griffith scholars generally make cases for his short films (A Corner in Wheat and The Musketeers of Pig Alley) rather than his features.
Uncanny Valley: Some of the "black" characters in the second part of the film almost fall into this territory due to the extensive use of Blackface. The eyes and eyebrows on some performers in particular look disturbingly out of place.
Unintentionally Sympathetic: We're supposed to take it for granted that Gus is trying to rape Flora (he was stalking her, though), but all we actually see him doing is asking her to marry him, then chase her to allegedly apologize, after which she promptly jumps off a cliff. This makes it hard not to feel sorry for him, Scary Black Man or not, especially given his punishment.
Values Dissonance: Though not to the degree one would expect, since both the film and the book it was based on were denounced by the NAACP as racist even at the time
The film follows the highly negative "Dunning School" view of Reconstruction. That approach was very popular among white Southerners at the time, but it has been rejected by almost all historians since the 1960s.