Diana's parents divorced when she was a young child and the resulting custody battle caused her much pain and anguish. According to CNN, divorce in the last 1960s in England was uncommon. It had a profound effect on Diana, who felt alienated from her classmates, especially since her father who was granted sole custody of Diana and her four siblings.
Kate hails from a far steadier family background than William's mother. In a time when divorce is much more common, they're still together and have been married for 31 years. Since Kate grew up with her parents' healthy relationship as a model, she may be better equipped for the ups and down of marriage.
Diana's childhood home was staffed with butlers and nannies, not to mention down the street from Queen Elizabeth II's Sandringham estate. Her father, a viscount, served the queen in the honored position of horse stable master, and the two families were friendly enough that Diana's future brothers-in-law—young Prince Andrew (Diana's teenage crush) and Prince Edward—would occasionally come over for a dip in the Spencer pool! When Diana became engaged to Charles, some people debated that the Spencers, whose ancestry and family money dates back to the 15th century, were more of a noble family than the Windsors, whose riches only date back to the 18th century.
Kate didn't grow up next to the queen, and neither did her parents. Her family is decidedly middle-class and title-less, much to the chagrin of some elitist Buckingham Palace staffers. Kate's parents, Michael and Carole, worked together as a team to build their (comparatively small) fortune. They met when they worked as a flight attendant and dispatcher, respectively, for British Airways. After they married, the couple started Party Pieces, a mail-order party supply company, that made them self-made millionaires. It's reported that someone at a Gloucester polo set said, "[Kate's] parents have a tarmac drive, for God's sake." Apparently, the upper classes prefer to ride on gravel. We're guessing Kate and William don't care.