Some very interesting comments and observations lucys, and much of it is spot on I would say.
I think most of the European princes and princesses barely rate a blip on the world stage, even when it is a very important or momentous occasion in their lives such as a wedding or the birth of the next heir. Very few royals that we follow avidly or discuss in such detail here are hardly known outside of their countries (or the German tabloids!
But I think there is even a sphere of relativity for which the British royals are known. Certainly the Queen, Prince Phillip, Prince Charles, Princess Diana and their sons Princes William and Harry, the Duchess of York are very well known on the world stage for good as well as infamous reasons. But the other members of the Queen's family, such as Princess Anne, Prince Andrew and the Earl and Countess of Wessex aren't as well known. (Prince Andrew keeps a very low profile so I can understand why so little is mentioned of him on the world stage, but I'm not sure I understand why the others don't receive much attention; maybe because there is very little scandal associated with them -- dog biting incidents and the fake Sheik incident aside.)
When Prince Joachim visited Toronto earlier this fall it was the first time many people realized that Denmark even had a monarchy -- let alone it being one of the oldest monarchies in history. I saw many of my colleagues with copious amounts of print outs from the Danish official web site as well as English translations of Danish articles about Prince Joachim.
An important point I think is that some royals are well known in certain parts of the world (even if it's just a city or a country) outside of their native country for assorted reasons. In Ottawa, where the Canadian parliament is, there is very much a soft spot for Queen Juliana of the Netherlands and her young family who stayed there temporarily during the war. Princess Margriet was in fact born in Ottawa but the land around the hospital where she was born was declared Dutch land so that Princess Margriet would be born a Dutch citizen. Following the family's return after the war, as a thank you, Queen Juliana sent thousands of tulip bulbs that were planted in the gardens around parliament. Several years ago, Princess Margriet returned to Ottawa to much fan fare.
And for other reasons certain royals just seem to capture the world stage, or parts of the world stage without any such emotional or significant ties as Margriet has with Ottawa. While Canada and specifically Toronto are known for their multiculturalness, and Spanish is spoken among parts of our population, it is not one of the top three languages (English, French and Chinese being the top three), yet when Felipe and Letizia became engaged in October of 2003, their engagement made the front page of one commuter paper as well as blurbs in all four local and national papers. I recently even found a picture of their wedding in the 'Just Married' section of our national weddings magazine, Wedding Bells.
Other royals break through for a moment but don't last long on the world stage. I remember when Haakon and Mette-Marit got married Mette-Marit's fairytale-esque story made it into a two page story in People magazine but nothing was mentioned again until the birth of Ingrid Alexandra.
And royals who are very much talked about here, I find hardly make any waves into media sources in North America, such as Maxima, Mary, Mathilde and Victoria.
To answer lucys question: I think the one royal who may have broken through the parameters of her own country would be Queen Rania. I think it's some combination of her being the youngest reigning queen, her beauty, her fashion sense and the fact that she comes from a Middle Eastern country is eternally fascinating. But to a lesser degree her work is much lesser known.
I think the world of royalty, despite the hard work of the various kings and queens, princes and princesses, will always be a relative and sheltered one. Unless they are your nation's monarchy or unless you avidly follow monarchies like all of us do here, you won't hear much about the likes of Silvia, Margrethe, Mary, Alexandra, Maxima, Sofia or Paola. And if you do hear about them outside of these circles, more often than not, it is for infamous reasons, such as if there was quite a scandal caused by someone that was so juicy and gossipy. (Although if Margarita's allegations of spying by the royal court and stories of her aunt Queen Beatrix being a boozer didn't warrant any stories on the world stage I can't imagine what else could be more scandalous.)