The biggest difference is the way Luhrman portrays Mercutio. Mercutio is a hang loose kind of guy and Luhrman took that a step too far with the drag queen act. The way Mercutio is played it's like he"s gay, and but he is just a party guy. A difference that is crucial to the plot is that at the party Juliet meets and dances with Paris. Juliet doesn't first meet Paris when she goes to confession, but already knew him. She knew of the proposal and the man before she knew Romeo. This leaves me wondering why she just didn't stay with Paris. The costumes worn at the party help to let the audience know more about the characters. Lady Capulet was Cleopatra and Cleopatra was a powerful leader. This shows that Lady Capulet is the leader in her family. Juliet wore white and angel wings. Juliet is still innocent of many things. Love is one of them. Romeo dressed as a knight showing that he is a fighter for what he wants. Towards the beginning of the party, Romeo took of his mask and threw it in the water. The mask was a symbol. Romeo was showing that he had nothing to hide by hiding behind nothing (a.k.a. the mask).
Mercutio's death was changed a little bit. The initial fight between Romeo and Tybalt was greatly exaggerated in the film. Romeo was not beat up by Tybalt, Mercutio challenged him to a fight before that could happen in the play. Obviously the weapons used by the duelers has changed. I think that the guns are a modern way to go, and I even think that the label was cute (SWORD .9 mm), but when Mercutio was slain, his wound was from a knife. That makes no sense. Luhrman could have done better there. The performance of Romeo was, personally, bad acting. Romeo was being a drama queen. The fight took place in the city streets but Luhrman has the setting changed to on Verona Beach. If the fight had been in the streets then the civilians would have been more involved and it would add in the factor of danger.