Certainly an unforgettably romantic story is presented in the play "Romeo and Juliet" by William Shakespeare. Many of the elements of a modern-day successful box office romance are present so let's look at those from a modern perspective. Firstly, youth - we know that the sweethearts are young and that Juliet ,at the beginning of the play, is too young to marry - so we find the sweet impulsiveness and passion of first true love very endearing. Secondly, there is beauty. We have all the imagery of Romeo's language to transport us away to a world of imagination where Juliet is like a jewel dangling on a soft cheek - and there is the beauty of the natural setting. There are also the barriers to their love - family injustice and hurdles which help both youngsters and parents alike to idnetify with the emotional pain and suffering of the characters and gets us rooting for them.
I don't know what Propp's morphology is (sorry), so I'm going with Frye's archetypes (see below).
Romeo and Juliet is only border-line tragedy, as far as I'm concerned. I would say that it's half tragedy, one fourth romance, and one fourth comedy.
Or, you can break it down by act: Act I and II are romance and comedy, and Acts III, IV, and V are tragedy. It's almost too plays in one.
And, you can break it down by character: Mercutio and the Nurse are in a comedy/romance, and Romeo and Juliet are in a tragedy.
According to Frye's archetypal characters, you can say that the play is a romance and that Romeo and Juliet are on a quest for love, though I wouldn't say anyone is ideal.
So, the Hero (Romeo) / Heronie (Juliet) are aided by Helpers on the Quest (Friar Lawrence / Nurse).
Enemies of the Quest are not your typical giants, orgres, or evil madmen. It is Tybalt, obviously. But also Fate ("star-crossed lovers") and the families' Hate, which could be the same thing, really.
Sprits of nature are the lark and the nightingale from Romeo and Juliet's bedroom scene and all of the light, dark, night, and day imagery. Enough already!
Realistic Counter Companion who calls attention to fear, the jester who deflates romantic ideals is Mercutio, obviously. His Queen Mab speech is an attempt to derail or counter the love quest.