The Nurse in Shakespeare's play is set up as a foil to Juliet. A foil in Shakespearean drama is a character who has an opposite personality and attitude to the main character. While for much of the play the Nurse is Juliet's prime confidante and friend, they part ways because the Nurse fails to understand the true love the girl feels for Romeo.
Being older and from the lower class the Nurse looks at things in a different way. She is more practical and much less romantic than the young Juliet. She looks at love in a more physical way and less as an ideal. When we first meet the Nurse she goes on about a time when Juliet, then a small child, fell down. The Nurse's late husband commented that when Juliet grew older she would like falling down on her back, a reference to sex. She tells Juliet that Paris will make her "bigger," meaning pregnant. For the Nurse, marriage and love are tied up with the physical aspects of life. After all, she is a simple servant.
The Nurse looks at Paris as simply a good looking man. She could care less about his personality. She comments in Act I, Scene 3:
A man, young lady—lady, such a man
As all the world—why, he’s a man of wax.
And later she refers to him as a "flower." She's only concerned that Paris is handsome and will make a good lover. She does not consider the romantic aspects of love. Romantic qualities, however, are what Juliet yearns for and once she meets Romeo and he tells her he is worshipping her as a religious pilgrim might, she is hooked.
The Nurse also doesn't have the type of loyalty that Juliet displays. As soon as things go badly with Romeo, the Nurse advises Juliet to forget him and marry Paris. At this point Juliet breaks with the nurse. She says in Act III, Scene 6,
Ancient damnation, O most wicked fiend!
Is it more sin to wish me thus forsworn
Or to dispraise my lord with that same tongue
Which she hath praised him with above compare
So many thousand times? Go, counselor.
Thou and my bosom henceforth shall be twain.
Above all, Juliet is loyal. She is loyal to her parents in the beginning and later to Romeo, so much so that she drinks a sleeping potion and fakes her death to be reunited with him. The Nurse, on the other hand, would quickly give up on the love the two young people share.
Pat Heywood, a Scottish actress, is still the best Nurse. She is perfect in the role in Franco Zeffirelli's 1968 version of the play. In Baz Luhrmann's 1996 version he updates the play to a modern time and makes Juliet's Nurse Latino, played by Miriam Margolyes, who is also excellent and steals every scene she is in.
If I were to cast a new Romeo and Juliet
I might pick Katy Sagal as the Nurse. She is most remembered for playing Peggy Bundy in "Married with Children." She can be bawdy and is now old enough to be convincing in the role. Another possibility might be Susan Sarandon, who's also older now and is one of the finest actresses in Hollywood.