One significant difference between Trifles and other murder mysteries is that the focus of the story is not on the investigators, but on two bystanders who normally wouldn't have anything to do in the story. The investigation is almost unnecessary to the actual story, which comes out incidentally through conversation. In this manner, the "trifles" of the title turn out to be far more important than the clues that the investigators are searching for.
COUNTY ATTORNEY: No, Peters, it's all perfectly clear except a reason for doing it. But you know juries when it comes to women. If there was some definite thing. Something to show -- something to make a story about -- a thing that would connect up with this strange way of doing it--
(Glaspell, Trifles, etext.virginia.edu)
A typical murder mystery would have more intrigue, more characters, and the investigating men would be more intelligent. The solution to the mystery would be discovered after gunfights and fistfights, and perhaps another death or two; the women would be there to support the men and for little other reason. By contrast, Trifles has no violence of any sort, and no action beyond conversation and general moving around. This makes it a cerebral mystery, rather than a visceral mystery.