In "The Gift of the Magi" what is so precious about Jim and Della?
Their love is completely pure, and they are willing to both sacrifice what is most precious to them in order to show that love. That indeed is endearing, and makes Jim and Della very special. A lot of people wouldn't go to such lengths to show their love. Della cuts off her entire head of hair for Jim, and Jim sells his beloved and treasured pocketwatch. And, they don't do it to show off, but to give each other a gift that they truly feel the other person deserves. Consider the following passage, where Della ponders her feelings for her husband:
"Her Jim. Many a happy hour she had spent planning for something nice for him. Something fine and rare and sterling—something just a little bit near to being worthy of the honor of being owned by Jim."
This passage shows just how pure her love for Jim is, and how pure her motivations for buying a gift. Often, we buy gifts in order to show off, or to get people to like us better, or out of obligation. But here, she wants to guy him one because she loves him so much that she feels he deserves one just for being wonderful.
That purity of motivation, and the lengths that Jim and Della go to show it, are what make them precious.