Matthew Yglesias gives some first-hand descriptions of stereotypical behavior on the part of tourists of various nationalities. While there is some truth to at least some of these stereotypes, the behavior of tourists is also greatly affected by (1) the type of tourists visiting a particular locale, and (2) the purpose of their visit. Hardworking, middle-aged non-cosmopolitan types on vacation to relax at the beach with their families are bound to act differently--and thus to reflect differently on their nationality--from, say, graduate students travelling in order to learn about foreign cultures. The same holds for teenagers or young adults vacationing from school or work--not to mention soldiers on leave.
For example, Canadians have a reputation in Europe for quiet politeness (particularly in comparison with their American cousins). That's probably because while Americans seem to view Europe as something of a theme park, Canadians are more likely to see travel there as an enlightening experience. On the other hand, I'm given to understand that in the Dominican Republic (a popular beach destination for budget-conscious Canadians) we're known as loud, obnoxious drunks.
The German and British reputations in Southern Europe no doubt have similar roots. I also once saw a busload of middle-aged French tourists in Seville, Spain, and from a distance I easily identified them--as American. Even up close, I had to listen to them to remind myself they weren't from the Midwest. I don't happen to know the fashionable destinations for broadminded travellers from these countries, but I would bet that wherever they go, their nationality has a pretty good reputation.