The criticism was due to the fact that (I'm going to repeat what posters have already said, in order to reinforce my point), (a) William and Catherine were said to have turned down a skiing holiday due to the economy crisis, only to go on a trip to an exotic place (even if the trip was being funded by the Middletons) a few weeks later, (b) they were not seen doing much concerning to royal duties previously. Therefore a holiday seemed a little too much given that time in space.
The other Europeans are full time royals (with its exceptions). You can see them out and about, attending engagements, pretty much every week, throughout the year. So, a family holiday can be understandable. Besides, I don't think we have seen any mixed signals coming from them. Like, avoiding doing a certain thing and then doing another similar one. So, at least, there's some consistency in their words.
Also, most of the other European royals have children. Therefore, as most families in the world, they need to schedule their family vacation around their children's school schedule too. So when they have their family vacation, it's an opportunity they have to be together as a family and do things together. Unlike, a married couple with no kids yet, who can afford to spend time together either at home or at a vacation.
(Not criticizing William and Catherine's right to vacation, though.)The BRF are always known to spend the summer at Balmoral or Scotland in general while their counterparts go to other destinations usually exotic.
That maybe has to do with the generations. The older generation of the Danish Royal Family (The Queen and The Prince Consort), for example, usually spend their vacations at royal summer palaces, or their private estate in France, while the younger generation (The Queen's children and their families) usually go away, "privately", to a chosen destination for a few days, and then return to spend the rest of the summer, more "publicly", with the relatives at the chosen palace.
