This writeup is about the single most potent spoiler in this movie. It's likely to surprise people who are quite familiar with the play. I entreat you to see the film before you continue.
Luhrmann has always been a daring filmmaker in the sense that all of his work is so broad and campy that it risks absurdity. Romeo + Juliet is his most uneven picture, with a number of sequences that simply don't click the way they should. In my mind, though, all is forgiven in the last fifteen minutes, which contain his most perversely successful gamble.
Juliet wakes up at the exact moment Romeo swallows the poison. She watches him die, and he dies knowing she's alive.
I have no idea whether this twist is original to Luhrmann, but that's hardly the point. The impact of this reinterpretation-- this subversion-- of Shakespeare's climax is unforgettable. Nor does it depend entirely on the element of surprise: Repeated viewings only make it more resonant.
It's also the quietest scene of Luhrmann's career. He puts no swelling music under it, and his camera is far less mobile than usual. The rest of the film, with all its noise, rapid cuts, and gratuitous head trips, begins to make sense as an elaborate preparation for an ending that might have been less effective without the contrast. Think of it as a cinematic variation on the second half of Mahler's ninth symphony, with its clangy, chaotic third movement setting up the steady, deeply moving fourth. Or you can just look at it as a rare moment of grace from an enjoyable but abidingly unsubtle director. It's genius either way.
Addendum: According to some—including Timeshredder, who brought it to my attention—this twist was first written into Romeo and Juliet in the 18th century. Whatever its origins, Romeo's late realization now seems integral to the play. In a production I saw recently at the Seattle Shakespeare Company's theatre, Romeo sees Juliet's hands twitch a moment after he quaffs the poison. It's profoundly appropriate that both of them, not just Juliet, should know the extent to which their plans have gone wrong. | 
