Far from a critic of or a student of art history, I will refrain from supplying yet another interpretation of this portrait. I'm not quite sure that this result was what the artist, however talented, was aiming for, to say the least.
The British monarchy has had a long history of official (and unofficial) portraiture. Nothing has changed the nature of portraiture more than the increasingly widespread use of photography. Although portraits of monarchs and their families continue to be painted to this day, photographs have overtaken paintings in crafting images of the British monarchy. Arguably, because of photography, portrait painters are forced to create far more candid, true-to-life images than their image-conscious predecessors. Instead of portraying the monarch as he/she wished to be, portrait painters now attempt to create images that are as close as possible to how their subjects actually appear and behave.
Here is an image (more famous) of her family in 1857:
After years of reports of scandalous indulgences perpetrated by the previous generation of her family, Victoria and Albert came to represent stability, devotion to family, a new morality, and what would later become a Victorian perspective of the roles of men and women in home and society (which, sadly, continues to influence how historians today interpret pre-Victorian ages). Images like the one above contributed massively to that campaign.
Going back further, arguably the Tudor family deserves the most credit for creating an image of monarchy portrayed through portraiture. The portrait to the right is of--yes, you guessed it--King Henry VIII. Although many historians have attempted to interpret Henry's personality, it is clear that he was a rather vain man who was somewhat obsessed with his physical appearance and how others perceived his vigor and virility. This portrait was painted by Hans Holbein the Younger around 1537. Henry had been through two wives already, was married to his third, and there were three more to come before his death a decade later. Because he hadn't produced many living children--not to mention his lack of a male heir--there had been a whisper or two about just how "manly" this king really was. The portrait gains additional significance depending upon when in 1537 it was actually painted. Jane Seymour was pregnant throughout most of 1537, and, unlike her predecessor Anne Boleyn, managed to carry the child to term successfully. In October, she gave birth to the only male child who would outlive Henry, the future Edward VI. Jane's pregnancy, and perhaps even the birth of his son if this were painted that late in the year, could have been the inspiration for such a confident, masculine, powerful portrayal of Henry VIII.
Who was the first English monarch to have his/her likeness captured? That is a difficult question to answer. Images of monarchs appear in a number of manuscripts throughout the Medieval period, but it is generally agreed that these were not taken from life. It is believed by scholars that some Medieval sculpture, primarily in churches and cathedrals, does portray important royal figures. However, the first royal portrait taken from life is believed to be of King Richard II.
There are several images of Richard II that were painted during his lifetime. One of the most famous appears on the Wilton Diptych, which is in the National Gallery in London. This devotional set of panels was painted in the tradition of altars, both stationary and portable, that were popular in the 14th and 15th centuries in Europe. It portrays Richard II kneeling before Mary, holding an infant Jesus, who is offering Richard a blessing. The portrait above, showing Richard II seated with crown, scepter, and orb, was painted between 5 and 10 years earlier around 1390. The timing of this portrait is therefore significant in the context of the many problems that occurred in Richard's reign. In the late 1380s, Richard had been forced by Parliament to condemn several of his most important court allies. By 1390, Richard was eager to reassert his royal prerogative. Richard took control of the British government formally in 1389, having come of age to do so. Although the chroniclers seem to classify Richard as a less-than-ideal king, he remained in control through the 1390s, and he even attempted to redress the wrongs perpetrated against him in the 1380s. Richard's story did not end happily. Richard was overthrown by the future Henry IV and died in captivity, perhaps by starvation, by 1400.
How will Kate's famous portrait hold up in the succession of royal images? It's difficult to say at this stage, but there's no doubt that it is a part of a long history that will continue for generations to come.