Aside from the ongoing political debates that D.W. Griffith's films fed into, and off of, his major impact on culture is that of the Feature Film, or a standalone, long-form film instead of shorts or series. Griffith also pioneered or invented many filmic techniques that are still used today.
From a political standpoint, Griffith's landmark work Birth of a Nation (1915) was the first to have a major impact on culture. At the time of release, the Ku Klux Klan had become a mostly fringe group, without popular support; the massive success of Nation allowed the Klan to reform as a politically and culturally influential force. Bolstered by public support from President Woodrow Wilson, the Klan was able to shape the civil rights debate for the next fifty years, helping keep Jim Crow laws in place and keeping blacks segregated and marginalized.
From a filmic standpoint, Nation was the first "blockbuster" film, successful across the country, and endorsed by the President. It created or standardized techniques such as the fish-eye lens view, the close-up, and expensive special effects, which before that time were limited in scope. Running over three hours, it set a standard for long films instead of shorts; until the rise of television, feature films became the norm for film entertainment, with serialized films running even longer. It became normal for people across all classes to have seen and appreciated the same work, instead of seeing only locally-produced films, and no other film of Griffith's has had the same impact (not even Intolerance (1916), which he released to placate audiences who criticized Nation as racist).