What makes Trifles such a great play when some would consider the fact that it is a One Act play as a negative point?
One way to approach this question might be to analyse the various themes of this play and see how they are defined and established to discuss whether or not Glaspell succeeds in her task of writing an exceptional play, albeit in only one Act. I guess you could treat this question as being similar to asking why some short stories are so effective, in spite of having the length that other, longer novels have. Certainly, One Act plays, just as short stories, need to do what they do quicker rather than the leisurely way longer plays are able to introduce characters and conflict, but this play certainly succeeds in presenting us in the middle of a tense situation.
To me, one of the most moving elements of this play, that certainly indicates how effective it is, comes at the very end of the play, when Mrs. Peters and Mrs. Hale conspire to hide a vital bit of evidence from the men in an act of female solidarity with Minnie Wright. What is surprising is that it is Mrs. Peters who first tries to hide the dead bird, as she at the beginning of the play seemed to be very much of the opinion that the menfolk were the experts and women shouldn't meddle in their work. This represents a huge success against male patriarchy, which is, of course, one of the central themes of the entire play.