In the fourth century BCE, the Greek philosopher and scientist Aristotle attempted to define tragedy in his Poetics. One of the areas Aristotle attempts to define is the tragic hero. According to Aristotle, the tragic hero
If we attempted to apply this definition to Shakespeare's tragedy of Romeo and Juliet, we could see how both Romeo and Juliet could fit the bill, because they both come from two of the most prosperous and famous families in Verona.
As for Willy Loman, he is not "highly renowned", he is not "prosperous", nor does he come from a famous family. Willy Loman, as his last name indicates, is a "low man." He embodies the average American of the 20th century, a man struggling to provide for himself and his family.
Aristotle also says that the hero should move from good fortune to bad in the best-constructed sorts of tragedies. In Shakespeare's tragedy, Romeo and Juliet are prosperous and have few worries; by the end of the play, though, they are both dead.
In Miller's play, Loman goes from bad fortune to worse. Early in the play we learn that he has been trying to kill himself. At the end of the play, he finally succeeds. Ironically, Loman kills himself before he can find out that his wife has finally paid off their house and that they are free of debt.