I've spent A LOT of time reading posts on this board as well as the Graffiti Wall and have concluded that the worry about what to wear in Europe is really stemming from the numerous reports of pickpockets and scam artists in Europe. So, perhaps Americans are concerned about being a target and are trying to find a solution/defense/security through the clothing they wear.
Yes, yes, I know, pickpockets exists and target tourists all over the world, but I can't help but notice how there are so many more reports about Paris, Rome, and The Continent in general compared to the UK. I think I've read all of 3 reports of pickpockets and/or scams in London.
And yes, yes, I know, if you act like a tourist, everyone will know no matter what you are wearing. I think at this point we need to start threads discussing tricks to not acting like a tourist.
Plan ahead. Study your map and where you plan to go before you leave your hotel. Fold the map so that you can easily see the part of the city you are planning to visit that day. Figure out surrupticious ways to peek at your map. The main thing I've learned, and I'm sure many people here know this, look and act like you know where you're going. In my experience, thieves or "bad guys" in general see any hesitation as an opportunity.
However, not everyone is adept at this surrupticious behaviour. Perhaps they've never been in a situation in which such behaviour has been necessary. Perhaps they just need stories of some of the tricks seasoned travelers have developed to not "look like a tourist." Otherwise, they may see "what they wear" as the only way to "defend" themselves from what seems like hords of scam artists and pickpockets that the countries of Europe let run wild.