The majority of the plot of Twelve Angry Men focuses around the biases that cause the men to vote the way they do. It seems that Juror #8 is the only one to put his biases aside to look at the case objectively.
For instance, Juror #10 is racist. He believes that "those people" (the racial, ethnic, sexual or political group that the young boy belongs to) have no value for human life, are always killing and up to no good. He lets this pre-judgement color the facts of the trial for him.
Juror #3 lets his personal relationship with his son color his votes about the boy's guilt or innocence. Juror #3 has unresolved issues with his own son, and is therefore determined that his boy will pay for the hurt his own son caused him.
Juror #9 allows his bias toward the eldery (and himself) to allow Juror #8 to continue on. Juror #9 feels he understands the old man (being old himself) and allows himself to empathize with the old man. Juror #5, being from the slums himself, is also able to empathize with the boy on trial.