Here in Belgium, American and the Japanese tourists and expats have the reputation for being 'high maintenance', and asking much more questions about every little practical detail than anybody else (instead of trying to find out by themselves). I am pretty sure that is because the USA and Japan are service societies, and people there are used to have everything done for them by professionals. Europeans are very different in that regard, much more independent-minded. Whether they are from the north or the south, from west or the east, Europeans don't vary much on this point. Europeans prefer to do things themselves than to have to pay for someone to do it for them. That's why relocation services in Europe are mostly for Americans, Japanese, and perhaps other Asians (China, India, Emirates).
American and Japanese tourists to Europe are also much more likely to travel in groups (organised tours) than by themselves. Once again, the service society... I noticed that people travelling in group (whatever the nationality) tend to be noisier (individually) and care less about locals and local sensitivities, probably because they feel protected by the group and form a sort of 'cultural bubble' impervious to their surrounding. Someone travelling alone will have to "fit in", to observe locals and try to speak the country's language. Within a group of compatriots with a guide doing the communication with the locals for you, you can behave just like in your country and not care about where you are.
As far as individual travellers are concerned, I have noticed that the Americans and the French are much more likely to complain about all sorts of things. This is something that they also do a lot in their country (the French are râleurs* by nature). But it may also be due to the fact that they both regard their culture as superior to anybody else, and consequently feel entitled to tell others how they should do things.
The Italians tend to compare everything to Italy and are a bit too set in their ways and habits, which makes them rather inflexible when travelling abroad.
The Indians can be annoying because they like to negotiate for everything and take their time to make a decision. They are usually not very familiar with European cultures and are more likely to break cultural conventions and taboos (but not as much as the Chinese, who really live on their own planet).
* there isn't a proper English translation. Grumpy, complaining about everything all the time.