This film is highly notable for its visual effects and amazing photography. The use of soft filters, haze, a precise balance of light and dark values in the image, with blooming coming from highlights along the edges was revolutionary at the time in regards to cinematography.
This film heavily influenced Ingmar Bergman, co-opting a sequence of a man seeing a clock, a passing carriage, and then himself in a coffin into Wild Strawberries.
There are a lot of trick shots, which given the circumstances by which they were used were basically being invented at this time. We see a lot of double exposure effects, which were done extremely convincingly and this helped set a precedent for later filmmakers to use effects as visual motifs or central elements
These early films are really pioneers in establishing what can or can't be done with the medium. Birth of a Nation and Intolerance establish the use of grandiose sets and storylines in establishing the epic, The cabinet of dr. caligari in establishing visual style and horror, and The Phantom Carriage in establishing cinematography techniques and visual tricks to tell a story.
On a story level, the film uses flashbacks within flashbacks in a way that no one else had really done yet, pioneering yet another groundbreaking technique that filmmakers later would use and develop further.