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After hitting the red carpet Sunday night for the London premiere of Steven Spielberg's film, "War Horse," Buckinhgam Palace says,  Kate Middleton celebrated her 30th birthday today with a "low-key and private" event.

It was the kind of quiet day Diana, the Princess of Wales, desperately wanted for her 30th birthday, but her desire for a subdued affair in 1991 was the result of despair, rather than respect.

She was unhappy in her marriage and refused to let Prince Charles throw a birthday party for her, British papers reported. Although Diana had requested a "business as usual" day, she was serenaded by Phil Collins at a charity lunch and the crowd outside the event sang happy birthday, according to the Press Association. Diana reportedly said later that she was "embarrassed by the fuss."

Diana was even quoted in one newspaper as saying she would toast her birthday with son Prince Harry, who was 6 at the time.

"I shall have a drink with Harry at 6 p.m.," she reportedly said. "He is the only man in my life tonight."

Katie Nicholl, a royals reporter for the Daily Mail, told ABC News that the most striking contrast between Diana and Kate at 30 is the strength of the royal marriage.

"I think the most important difference is that [Kate] is deeply in love with her husband, very happily married and she's been very warmly welcomed into the royal family," Nicholl said." [Diana] was nine years into her marriage, desperately unhappy, felt alienated and isolated - not just in her marriage, but also in the royal family. … The public adored her, but the greatest sadness was with how miserable she was in her marriage."

By the time Diana was 30, she was a mother of two, traveling the world pursing a portfolio of philanthropic causes. Kate has yet to have a child and is just beginning her public life.

She recently announced that she will be donating her time and support to an array of charitable organizations that include a group of children's hospices, an art therapy and addiction charity, a world-famous art gallery and the Scout Association, the British equivalent of the Girl Scouts.

"The problem with Diana was that she didn't play by the royal rule book. She did many great things and shook the royal family up, but in doing so and breaking the mold she roughed people up the wrong way," Nicholl told ABC News. "I think Kate has done things very differently. She followed the royal rule book literally rule by rule. She hasn't put a foot wrong, she has followed all the advice, she's followed her husband's lead and she's proving to be a great success."

The Associated Press contributed to this report.