AOG
If you have Netflix, there's a very interesting documentary series called 'The Royals'.
Each episode deals with a different facet of the royal family (Babies, Marriages, Funerals, Teens, Scandals and oddly - Pets).
The 'Funerals' episode largely deals with Diana, and watching it, I began to remember why she was such an important figure.
It's not true to say that she had 'total admiration', but I would argue that she was seen by many as being 'one of us'. She was like a bridge between the royals and us commoners.
There were traits that she had, which had not been seen in the royal family for some time - she was glamorous like the young Princess Margaret or Prince William (not the current one, but Prince William of Gloucester - the one who died in the plane crash).
Arguably, she possessed the common touch...she didn't have the air of mystery and aloofness, which is something that other members of the royal family get accused of.
Also, and this is the big ticket in terms of enduring affection - she died relatively young.
In terms of enduring popularity, that's practically unbeatable - Marilyn Monroe, James Dean, Bob Marley, Amy Winehouse, Buddy Holly, JFK, Paul Walker, Jimmy Hendrix, Jim Morrison, Kurt Cobain.