I think any large group of people is trouble. If you're in a large group you are far less likely, IMHO, to try and learn about the local culture. You're a lot more likely to be loud and obnoxious(mob mentality). People traveling alone or in small groups want to fit in... I think that transcends nationality and is just a base human trait.
On the topic... As a British person who has met reasonable numbers of American tourists (and I would define anyone who has a home, a career or a job to go back to as a tourist not a traveller - I hate this "they are tourists, but I am a traveller" attitude) I don't recognise much of the behaviour that the guidelines seem to imply. In my experience, Americans abroad are not especially loud-mouthed, small-minded, ignorant or rude. If anything, I'd say that Americans are slightly more likely to have a rather exaggerated courtesy and willingness to talk to strangers. The guidelines sound to me as if what's really being recommended is that Americans try and be discreet about their American-ness and not make it too obvious. Also, a slightly off-topic thought that occurred to me while I was thinking about this - has anyone else noticed that any arbitrarily selected collection of people - people of one nationality, say, or who work for one company, or of one race, or from one town, or whatever - any group like that contains a similar proportion of wankers?[British idiom: wankers = arseholes] It's like a scientific constant. I haven't got around to measuring it yet - maybe 10%?
skip with regards to some of the replies you can lead a horse to water but.......... don't get disheartened
A smile in any language is still a smile. Be nice, be courteous, share, and be respectful of everyone regarless of their nationality. Loud, obnoxious, aggressive people come in all nationalities. The perception of the "Ugly American" can be eliminated, let it begin with me.