Like one of the posters above, I also am struck by the light and dark imagery in the story along with the notion of blindness. These motifs dominate the story, rendering the ideas symbolic and meaningful.
In the end, it is the lights going out in the hall that brings the boy his personal illumination. He is accustomed to standing in the shadows where he and his friends pretend to be characters out of a fantasy. When the lights go out in the hall of the bazaar, he realizes that is what he has been doing with his crush on Mangan's sister - just playing a part in a fantasy.
Point of view is of paramount importance in this story, as well as the plot. Told from the boy's perspective, his idealism seems less silly and the crushing blow of his epiphany is all the more effective as he describes his feelings at the moment of disillusionment.
I like the allusions/symbols in the story. The jars at the bazaar stand like guards (like the angels who cast Adam and Eve out of Eden); the bazaar is like the forbidden Garden of Eden which is denied him (hence the guards); Mangan's sister is the forbidden fruit put behind the gate/fence that he is unable to pass. Cool story.
You will have to make the case for what the most important literary element is, but the light and dark imagery always strikes me when I read the story. When the story opens the boys are playing on a dark street, but the sister of his friend is illuminated by the light and she seems to almost glow. There are several examples of this device throughout the story and that imagery connects to the thematic point of the boy's "loss of innocence" as he realizes that the bazaar and the girl aren't what they seem. Not all light/bright things are what they seem.
This story features a crushing moment of epiphany, which is when a character realises a central truth about themselves and their life. We are introduced to the narrator who, in a childlike way, imagines everything to be incredibly Romantic - he sees himself as going on a quest like a knight to bring back a token from the bazaar for Mangan's sister. However, note how the journey begins to reveal to him that the bazaar is anything but the Romantic location that he thinks it is and when he finally gets there he realises how he has been kidding himself. He grows up, realising his own "vanity."
The most important element of this story is plot. Without it, the story would not be able to exist. The plot of the narrator to go to the fair in search of a present for his illusion infatuation is what drives the story.