A few things that I think make "One Week" great:
As with all Keaton films, special effects are minimal and most stunts are performed by the stars. What you see is what happened: the cars, the house, the trains are all real. I think that gives the viewing experience an authenticity, immediacy, and tension that no computer-generated effects can match. They literally couldn't make 'em like this today.
The young married couple played by Keaton and Sybil Seely is very sweet and feels completely modern to me. I buy their relationship; they're in this mess together. Seely is particularly charming: smart, vivacious, sexy, and an equal partner to Keaton's hapless groom.
In 2008, "One Week" was added to the Library of Congress's National Film Registry, marking it as a "culturally, historically or aesthetically significant work to be preserved for all time." I agree. I know silent films aren't everyone's cup of tea, but if you're not one of those people I think this one's a treasure and worth a look.