Nineteen-year-old Rachel Jeantel holds some of the most critical information about the Trayvon Martin murder case. Yet her delivery on the stand in Florida's Seminole County this week drew widespread criticism.

She was hard to understand, mumbled, acted impertinent, annoyed, rude, and came across, as one cable TV news host said, as a “train wreck.”

But the torrent of negative reaction across bar stools and Twitter became more telling than Ms. Jeantel’s simple testimony relating what she heard on the phone as she talked to Trayvon before the sound of a “thud” on wet grass and a disconnected line. Moments later, Trayvon, an unarmed black youth, was dead from a single bullet from a 9mm Kel-Tec pistol registered to George Zimmerman.

While some have rushed to defend Jeantel’s multilingual background, others leaned hard into her personally, letting fly on social media a swirl of epithets that roughly amounted to dismissal of her as “ghetto trash,” as one commenter said. That reaction has steered the trial into a new phase, reflecting, some commentators argue, more on America’s privileged classes, including blacks, than Jeantel’s trustworthiness as a star witness.

Reaction to Rachel Jeantel on the stand “has been in terms of aesthetics, of disregarding a witness on the basis of how she talks, how good she is at reading and writing,” says George Ciccariello-Maher, a history and politics professor at Drexel University, in Philadelphia. “These are subtle things that echo literacy testing at the polls, echo the question of whether black Americans can testify against white people, of being always suspect in their testimony. It’s the same old dynamics emerging in a very different guise.”

To be sure, in the scathing commentary about what some called her puzzling demeanor and alleged lack of education was lost her singular background and her youth: A black and Creole girl growing up in a segregated Miami community, she represented part of the problem of the case – an America so divided, that many can’t “code-switch,” or move between the gauzy racial, cultural, and socioeconomic divides that have become hardened with the nation’s first black president, and which have helped fuel political polarization.

“What so much of this really revealed was the gulf between middle-age, middle-class, mainstream codes of behavior and life among youth from poorer, nonwhite neighborhoods … they couldn’t have been further apart if Jeantel were born on the moon,” writes Eric Deggans in the Tampa Bay Times.

That divide epitomizes the trial itself, Mr. Deggans argues: “As each side on this murder trial tries to prove the other person had tendencies toward prejudice and violence that may have sparked the fight, how will jurors [five white women and one Hispanic woman] judge the difference between edgy culture and outright dysfunction?”

Jeantel spent nearly seven hours on the stand over two days, relating some of the most riveting bits of information about the night Trayvon died, crucial to the case. While others have said they saw the teenager beat Zimmerman, that came only after Jeantel said she heard a heavy-breathing man, allegedly Zimmerman, say to him, “What are you doing around here?” and after Trayvon told her that a “creepy-ass cracker” was following him.

The state alleges that Zimmerman profiled Trayvon, who was returning to his father’s home with a bag of Skittles candy, a can of iced tea, and $40 in his pocket.

The state says Zimmerman chased and confronted Trayvon, and then fired at him only after he realized he was losing the ensuing fight. Zimmerman says he fired in self-defense after the teenager doubled back and attacked him, breaking his nose and bashing his head on the sidewalk.

The killing became a national story after Sanford police refused to charge Zimmerman with a crime, saying they had no evidence to counter his self-defense claim. Forty-four days later, prosecutors finally indicted Zimmerman on second-degree murder charges. If convicted, Zimmerman, an aspiring police officer who served as a neighborhood watch captain, could spend the rest of his life in Florida state prison.

The big question hanging over the trial is whether it was an unarmed Trayvon Martin who claimed his self-defense rights against an armed adult stranger following him in the dark, and whether Zimmerman waived his self-defense rights when he made the decision to pursue Trayvon after noting to a 911 dispatcher that “these [guys] always get away.”

Yet the potential for Jeantel’s testimony to illuminate that central question appeared to sink beneath a wave of commentary about aesthetics, as Christina Coleman summarizes in a Global Grind article called “Why Black People Understand Rachel Jeantel.”

“I … understand why white people wouldn’t like Rachel.… But maybe the reason white people don’t understand Rachel Jeantel has something more to do with white privilege than what they could call Jeantel’s capricious nature,” she wrote.

But Ms. Coleman’s contention that jurors should accept that blacks and whites often live in different worlds rather than as equal members of a polyglot American society is a problematic explanation, writes J. Christian Adams on the Pajamas Media website.

“Coleman sounds like John C. Calhoun, the South’s leading defender of slavery and segregation,” he writes. “Calhoun believed that blacks and whites could never live together, and that after any emancipation they’d forever be ‘worlds apart.’ ”