Kelly's great review below begs the question: What does it mean that love is forbidden? Can we rightly compare ROMEO AND JULIET with TWILIGHT? Are they talking about the same thing?
To this end, a little brief, historical comparison...
Let's go medieval for a moment, with A KNIGHT'S TALE. Having re-watched this with the folks recently, I was struck by the ways in which, once again, this is a tale of forbidden love--a humble thatcher masquerading as a knight to gain the love of the royal maiden. Heath Ledger does a good job portraying William Thatcher, the knight-to-be, to Shanynn Sossaman's lady. BTW, whatever happened to her?
Anyway, the tension in this one exists because of societal boundaries: there's a fairly rigid 'caste' system of sorts, codified because of divine right of kings and centuries of tradition, such that people are what they are. However, in Chaucer's day, these rules are breaking down; knights are rising up to nobility; merchants are moving up into a new middle-class; royalty are no longer respected as royalty if they're total pricks. But the point is this: what makes the love between Heath and Shanynn 'forbidden' is a global device, a societal vision which is, in a sense, totalizing. There is no other Europe other than the one they live in, and so, if you're going to fall in love, you have to fall in love according to the rules of Europe, which means "no love across class lines".
Heath and Shanynn, thus, have to pursue their love, not in spite of the rules of society, but deceptively through the rules of society. Their love comes to completion only as Ledger ascends to knighthood and joins the ranks of nobility, and some sense, making their love no longer forbidden.
Fast forward a couple of centuries past Chaucer, and we come to the Grand Maul Seizure of forbidden love: ROMEO AND JULIET. Say what you want about the 1968 Zefirelli version; I thought the Baz Luhrman version with DiCaprio and Danes was phenomenal. But in any event, let's examine what it means here for love to be 'forbidden'.
The emphasis of R&J is on the family role of 'forbidden love'. These star-crossed lovers are kept apart, not by societal lines, as this is the 16th century; societal boundaries are really fluid; monarchies are dying off left and right, and the middle-classes are making their case to be the new nobilities. See Shakespeare's OTHELLO for the ultimate example of societal movement during this time. In R&J, what keeps the lovers apart is not societal regulation, but family regulation. In the absence of a firm, uniform world as in the Middle Ages, families become the new boundaries within which 'forbiddenness' can be established, more or less. In any other circumstances, barring the names of 'Montague' and 'Capulet', we have a comedy, but because of the regulation of love by family contraints, this one turns into tragedy.
Granted, 'family' depends in part on where in society they fall. Had Romeo, for example, been a commoner, Juliet's family would have objected for other reasons; thus, Paris is the perfect match in their eyes for Juliet, as he's the bachelor-of-the-month (played by a young Paul Rudd in Luhrman's version). But what I want to point out here is that the guiding rubric for 'forbidenness' is that of the family; the family has imbibed their understanding of acceptable love from society, to be sure, but as far as who adjudicates the boundaries of right love--this falls to the family and not society at large.
Moving on to the modern-day CAN'T HARDLY WAIT, one of the understated gems of the early 90s, we find the endgame of 'forbiddenness': the forbidden love posited by and against one's self. Whereas in KNIGHT'S TALE and ROMEO AND JULIET, love is forbidden by forces or persons external to the individual, by modern-day, the rules for true or forbidden are posited by the individual themselves, apart from family or societal considerations. Notice, for example, that you never see the main character's families in CHW....
God bless Jennifer Love Hewitt. In this film, both positively and negatively, the 'right' love is that which is given the character by themselves: Amanda, despite her societal standing, rejects the tool Mike Dexter in search of the real deal; Preston Meyers decides that Kurt Vonnegut is awesome literature and pursues the dream girl that he knows is the true love of his life, Amanda Beckett. The two are drawn together in spite of who? Not society, which they defy, nor in spite of families, which do not exist in this film, but in spite of themselves. Both Amanda and Preston struggle to hold faith to an ideal which exists outside their vision, and to deny the tendency to choose that which is before them, in pursuit of that which is beyond them. In other words, their greatest fight is the one that they have with themselves, to pursue love which they know is ridiculous, but which they desire anyway. Maybe a better example of this would be the Seth Green scenario in which he winds up with his ultimate nemesis, the wankster falling in love with the literature nerd, two souls finding each other, despite themselves.
I submit that NEW MOON is of this last kind, the great postmodern forbidden love story, in which the true oppressor is not society, not family, but ourselves. Granted, the stakes are slightly higher here: should love conquer the self, somebody's getting a brand new set of glittery skin for Christmas. But in the end, the lovers in NEW MOON have only themselves to answer to and to overcome in order to achieve their 'forbidden love'.