Skydragon said:
Frederick and Mary met in a bar through mutual friends. Therefore they already had some things in common.
I don't think you can see it that way. In the book 'Kronprinsesse Mary - fortæller til Anne Wolden-Ræthinge' the pre-wedding interviews with Mary Donaldson are published. You can find the translations here:
http://www.theroyalforums.com/forums/96917-post2.html
Mary tells about her meeting with Frederick:"I lived together with four others in a big house. It was at the time of the Olympic games in Sidney, and one of my flatmates had been invited to take a few friends along to a bar to meet some spaniards, who participated in the games."
Mary by chance joined the Australian group while Frederick by chance went with his cousin Felipe of Spain who had decided to join the Spanish group.
It was a one-time meeting, not one of a lot of meetings within a certain social circle. Mary was only acquainted with the one group, Frederick only acquainted with the other. Mary would never have met Frederick again if he had not asked her out to dinner immediately and started courting her.
What kept them together? Mary said: "Frederik and I began to converse, and we simply didn't stop talking. And that was IT! A very long conversation, which continued over a year or rather 14 month." Common grounds, indeed. In the absense of the conversation topics of "polite society" they obviously found things to really talk about.
It seems to be similar with Kate and William: they met by chance and obviously found common grounds. As it seems, they are still talking to each other and may continue so till they are both old and weathered.
At least I wish it for them - William's parents came from the same social background but that obviously didn't give them enough topics to talk about during their life together. I wish for William to find his sould mate, be it in a bar or someplace else. Maybe the place was the university and it has already happened.
As for the other points: I don't think that social status matters too much, as long as it is not too low. Here in Germany our society was tumbled around by the end of WWII, when lots and lots of formerly noble and rich families lost all and had to flee to the West. So here you find lots and lots of people whose ancestors had a grand past but who live perfectly ordinary lives on studying and working for their living. Each of those could easily marry "up" on getting the boss or his heiress sister. Or marrying the local prince or his visiting cousin. Probably it's different in the UK where this gigantic "Voelkerwanderung" did not happen, where society seems to be much more fixed. So I guess I subscribe to your views, as they make sense to me (apart from the Fred-Mary relationship, that is).