Not to be outdone by The Daily Mail and the New York Times, The Economist  has pronounced on the engagement of Britain's Prince William and his girlfriend of many years, Kate Middleton. 

"The monarchy needs a combination of stability and glamour," says the transAtlantic magazine. "It is a
tricky balance to maintain." On the stability side, Kate's stable family background should help:

Unlike Diana, who was disadvantaged not just by warring aristocratic
parents and a tiresome stepmother, but also by a faintly ludicrous
stepgrandmother who wrote 664 romantic novels and always wore pink, Miss
Middleton comes from a stable background. Her parents met working for
British Airways; she was brought up in a two-parent, three-child family.
Since people from stable families are more likely to produce them, she
has a better chance of making a go of her marriage than Diana had.

This contrasts with William's, and his mother's families. Moreover, Kate has age on her side: she is 28 compared with Diana's 20 at the time of her engagement to Prince Charles.

On the other hand the newly-engaged couple have one well-known risk factor for marital instability -- prior cohabitation. According to NYT:

By all accounts, Miss Middleton is tough and savvy, and far better
equipped to deal with media attention than Diana was. Also, while
Charles and Diana hardly knew each other at the time of their
engagement, Miss Middleton has been virtually living with Prince William
for some time and has met his father many times. The couple became
engaged in October during a private holiday in Kenya, Prince Charles’s
announcement said. 

One can only wish them well.