Watch your step! Dog... stuff
This is one of those things I'd like to say is just a silly xenophobic American stereotype of life in France, but there really is a ridiculous amount of dog shit — pardon my French — on the sidewalks and streets of Paris. Many Parisians simply don't pick up after their dogs. I'm told these days more and more people are starting to, and according to many Parisians it's not nearly as bad as it used to be, but if that's the case I wouldn't want to see how it used to be. You may notice signs everywhere begging people to pick up after their dogs, but many people just ignore them. In any case, you'll quickly learn to watch your step all the time. The streets in most parts of Paris are cleaned more often than we're used to in American cities, so this might account in part for why French people don't feel the need to clean up after their dogs, but that doesn't make it any less disgusting.
In any case, even if you don't see anything nasty on the street, it's very important to wipe your feet before going into your hotel room or anyone's home. An interesting observation: I've noticed that while maybe half of the American households I visit have a "no shoes in the house" rule, practically every French household has this rule, and I always suspect it's at least partly because of the dog shit. Anyway, watch your step!
Related junk from Cool Stuff in Paris:
Basic etiquette for Paris; la bise, etc.
What to wear (and what NOT to wear) during your stay in Paris
Paris weather: some general info
The arrondissements — Paris' 20 districts