From the Prologue we know Shakespeare wants to make this play a big deal. Besides the fact that he employs the Greek tragedy-esque Chorus, check out how epic his language is, right from the beginning:
Two households, both alike in dignity
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes
A pair of star-crossed lovers take their life…
We've got some mighty words here: ancient grudge, civil blood, fatal loins, star-crossed lovers. This language continues through the play. While such language could seem melodramatic, Shakespeare makes it work. The plot lives up to the hype, so to speak.
Besides choosing epic-sounding words, Shakespeare pens a slew of passionate exchanges. In addition to Romeo and Juliet's romantic moments, Shakespeare gives many other characters some passionate dialogue. We're thinking of Mercutio's repetition of "A plague a'both your houses!" (3.1), and Friar Laurence's "Ah, what an unkind hour, is guilty of this lamentable chance!" (5.3). A lot of serious stuff goes down during the play, so the passion seems appropriate.
While decidedly passionate, much of Shakespeare's language sounds downright poetic. That's because it's written in poetic verse. Expect for exchanges between servants and some bawdy jokes, Romeo and Juliet is written in iambic pentameter, Shakespeare's meter of choice. You may be thinking, "Iambic pentameter, with all those stresses and syllable counts… for an entire play? However many hundreds of lines that is?!" And to that we say, "Uh huh. Isn't Shakespeare a lyrical genius?"
So let's break down what iambic pentameter means. Let's start with the "pentameter" part. "Penta-" means five, so we know we've got five of whatever a "meter" is. A meter is a group of two syllables, or "feet." The two feet are either stressed or unstressed, in some sort of pattern, which gives the line that certain rhythm that makes the line sound "poetic." So let's see – if each meter has two feet (a.k.a. syllables), and we have five meters, then each of Romeo and Juliet's lines will have ten feet, or ten syllables. We got the pentameter part covered. Now, "iambic" is a stress pattern – actually, the most popular stress pattern. It dictates that the first foot in each meter must be unstressed, and the second must be stressed. You might wonder how you can tell an unstressed foot from a stressed one? The official explanation is that a stressed syllable gets the emphasis in a word – or, as the linguists say, the syllable takes "phonetic prominence."
The easiest way to explain this is to just sound it out. Let's look one of the Friar's early lines. He says, "Holy Saint Francis! What a change is here!" (2.3) Shakespeare chooses his words carefully, so if you pay close attention to what words you naturally put emphasis on, the lines come out sounding pretty good. Look at what syllables we hear ourselves naturally putting stress on (capital letters = stress):
hoLY | saint FRAN|cis! WHAT | a CHANGE | is HERE!
Iambic pentameter can get tricky, but always bring it back to the basics: five meters of two feet each that follow the stressed/unstressed pattern.