Though fundamentally unknowable, the queen can't be too distant and unapproachable. At the same time, she must somehow give the impression of being close to her people. Her private motto is, "I have to be seen to be believed."
Hence her many appearances opening hospitals, christening ships and inspecting military regiments, always in her trademark hat and pearls. In fact, when Britons are asked to explain their admiration of the queen, many talk about how diligent she is in dispatching her duties, how the "old girl" soldiers on despite her age. She is only the second British monarch to reach 60 years on the throne and celebrate a diamond jubilee; the first was her great-great-grandmother Victoria, 115 years ago.
"The queen has become popular because there she is, an elderly woman who's still working," Billig said. "That's a very down-to-earth assessment."
Popular is right: The left-wing Guardian, which is staunchly republican on its editorial pages, nonetheless published a front-page story last month on the record popularity that "brand Windsor" currently enjoys.
The queen consistently ranks as the most beloved representative of that brand; thousands of well-wishers greet her wherever she goes, eager for some sort of connection. The question is how well the royal family will be able to sustain that semi-magical appeal once she's gone.
Many of the queen's numerous progeny maintain just as taxing a regimen of charity work, royal "walkabouts" and other ornamental appearances.
Yet unlike her, they have all had the veil of mystery stripped away to some degree, often in the most unflattering of ways. Prince Charles' recorded phone conversation with his then-mistress, now-wife, Camilla Parker-Bowles, still induces winces more than 20 years later, particularly the royal references to feminine hygiene products.
That kind of exposure might be in keeping with modern celebrity, but doesn't necessarily suit a centuries-old monarchy.
"When [the queen] goes, it'll completely change. It'll turn the corner," said Daws of north Wales. "If you look now, they're very touchy-feely, whereas with her, there's this boundary around her."
Author Lacey said that even the regular-bloke-ish Prince William, the second in line whose wedding to Kate Middleton a year ago set off a breathless bout of royal mania, may discover to his regret later that it's possible for the monarchy to be too of-the-people.
"He and Kate are a glamorous couple who chose Los Angeles, of all places, as somewhere to stop on their first foreign tour. Will people feel the same about them in 20 years' time, or will they not feel too much familiarity?" Lacey said.
"That's the risk, or the challenge, that lies ahead for the monarchy: how to maintain its mystique in an age of popular access," Lacey said. "It's what distinguishes royal celebrities from all others."