There has been a lot of chatter on this subject. I have been thinking about it and have some input.
In Europe, I see people wearing a lot of weird stuff, and you can always tell who is a tourist. But my take on it is that my goal is not to not look like a tourist, but to fit into the atmosphere as much as possible. I also feel it is respectful to try to look as good (and comfortable) as possible for the "dress code" of the destination.
For example, in Provence, casual is the way to go. But in Paris I would want to dress up a bit--no jeans or better jeans, for example. No sneakers, but I would wear those to tramp around Normandy beaches.
I have seen people inappropriately dressed in places like Notre Dame or Reims cathedral: shorts, tank tops, flip flips, etc. I just feel like we tourists gove each other a bad name by our choices of clothing.
So, I applaud anyone who wants to try to not look like a tourist!