I'm surprised that no one has delved into this one. I'm sure there are a lot of strong opinions about this topic out there. For my two-cents worth, there are too many American tourists dressed a la Americana: It's not the jeans alone but the jeans and the t-shirts, bearing all kind of inane logos and sayings, and the dirty sneakers. I have been to London and Paris several times and it seems to me that most of the European travelers dress a little more elgantly, espcially when going to the museums or Westminster Abbey or the restaurants; the same is true at the Louvre or la Saint Chapelle or the restaurants. This is not to say that there aren't any sloppy Europeans travelers -- they're there, too; it's just seems that we Americans have a disproportionate share of them.

Nowadays, jeans are widespread attire in Europe, worn not only by tourists, but very frequently, by the younger local crowd. As a very personal preference, I preferred dark, casual pants, even black Levy's with a nice jacket. I wear jeans only on days where the sightseeeing involves climbing, rugged terrain, etc. My husband on the other hand, feels very comfortable and actually prefers jeans to dockers when in Europe.

Being a European woman myself (from Belgium), I can assure you that in Europe a large amount of the people (young people and a-bit-older like myself, I'm 44) wear jeans nowadays, even in an elegant city like Paris. We in Europe often wear jeans in combination with a nice shirt or jacket and classic shoes or boots.
When you intend to go to fancy restaurants, you better leave your jeans and sneakers in your hotel.