Once upon a time America was opposed to tyranny and therefore rebelled against the king. However, we have come to the place where we have embraced leaders and laws far more intrusive, obstructive, and dare I say, tyrannical than anything imposed by the Crown.
In this day where much of the American populace pays rapt attention to every fornicating starlet who decides to have an out of wedlock "accessory baby," I think applauding a highly visible couple on the birth of their baby within the bounds of marriage, who by all appearances try to be as normal as their circumstances allow, is commendable. As leaders they set an example, and in this case the example of William and Kate is one to be emulated.
As to the monarchy itself - I have to say, in sheer economic terms, the Brits benefit from having a head of state that actually makes them money rather than ours who depletes our coffers more and more each day.