In Romeo and Juliet, what are the differences and similarities between the characters of Romeo and Juliet?

I'm sure you'll get plenty of responses to this question,
as there is so much information in the play that can lead audiences to compare the two
characters.  Foremost, they are both young, from families that are "both alike in
dignity," and they are both capable of selfless love. 

In
Act 2, scene 2 (the balcony scene), we see evidence of a difference between the two. 
Here, Juliet is a little more level-headed than Romeo, who is easily carried away by his
poetic and idealistic views on romance.  When Romeo starts to make a promise to Juliet
("Lady, by yonder blessed moon I vow..."), she chides
him:

O, swear
not by the moon, th' inconstant moon

That monthly changes
in her circled orb,

Lest that thy love prove likewise
variable.

Essentially, Juliet
recognizes that while it might sound like a good idea to swear by the moon, the moon is
changing and inconstant.  So, Juliet tells Romeo not to swear by the moon unless his
love will change like the moon does.

When Romeo persists,
Juliet tells him that she is wary of the
relationship:

Although I joy in
thee,

I have
no joy of this contract tonight.

It is too rash, too
unadvised, too sudden,

Too like the lightning, which doth
cease to be

Ere one can say it
lightens.

Again, these lines
show a difference between Romeo and Juliet; she is a little more cautious than he is at
this point in the play.