Divergents are not the only elements of this story that diverge from the norm. That's why Tris matters.
Tobias and Tris are standing face-to-face in the control room of the Dauntless Compound. Tobias is "in a simulation," meaning that his mind is being controlled. He doesn't know Tris, and he's going to shoot her.
"'Tobias,' I say. 'It's me.' ... 'Tris,' he says, and it's him again. ... 'How did you do it?' I say. 'I don't know,' he says. 'I just heard your voice.'" (beginning of chapter 39)
And there it is. What's so great (and I do mean great) about this moment? It blows a hole in the princess fairy tale. Snow White, asleep in a coffin until her prince kisses her. Princess Aurora, the Sleeping Beauty awakened by a prince's kiss. So entrenched is this tired cliche that it's the first thing Simon and Jared Grace attempt in their struggle to awaken Mallory in The Spiderwick Chronicles. ("Ew," they say, because she is, after all, their sister.)
In Divergent, it's Tris's voice, not her kiss, that awakens Tobias, but the concept is exactly the same. A helpless, hapless character is completely stuck in limbo until "awakened" by the hero(ine)'s love. Tobias was utterly helpless until the voice of his beloved snapped him back to reality. Woke him up.
It's a great moment because at last, here's a girl whose romantic love is life-saving. When else has that happened? There's the trope of a mother or father pursuing an unsavory lifestyle until thoughts of his or her daughter snap the parent back to reality, but a young woman's sexual love as the redemptive catalyst? That's just not a common (to put it mildly) plot point.
Tris's love is more powerful than any spell. If you're lucky enough that she'll share it with you, her love will save the day.
You roar, Tris!