I am an expat American who lives and works in France, about 70km outside Paris. I've lived here about two years with my wife, ten year-old daughter, and our two dogs. We live in a house in the countryside with two cars. That is, we're not tourists or students; we're regular American expats living in France. That's going to make my answers different.

Some big differences:

• There's a very good national healthcare system and the French use it regularly. If you are sick go to the doctor. If you don't get better then go back. Doctors make decisions about appropriate care, not health insurance companies. Care decisions are not based on cost. (If you're a tourist and get sick here also go to the doctor: even without insurance it's really inexpensive - about the the same or less than the usual co-pay's back in the US).
• The French care way less about money and way more about doing things well. You might look at restaurants and think they could make a lot more money if they'd turn over tables faster or did some obvious things differently. They won't because that would erode the experience, in their eyes, and money isn't worth it.
• There is way more vacation and people are expected to take their vacations. It is mandatory to take a summer vacation that is at least two consecutive weeks in the summer. Not encouraged: it's part of the labor law.
• Like anybody from a big city Parisians are different than French elsewhere; think about the difference between Manhattanites and the rest of the US. Spend some time in Paris then get out! There is so much more to see. Rent a car and check out all the small villages and palaces and other things. Driving laws are pretty much the same as in the US so it's easy (except that when you approach an intersection where nobody has stop signs or stoplights the person on the right has a right of way - this makes you slow down on small streets when there's something on the right - and a blinking arrow at a red light functions like a green arrow in the US - that's about the only differences).
• Ignore what other people say: the French are warm and welcoming and helpful and overall kind. Of course there are jerks but there's jerks anywhere; it's less acceptable here. Speaking of which if something isn't going well you'll be much better off trying to resolve it from a place of kindness. For example, if you don't like your hotel room you're more likely to get a better one by explaining you saved for a trip to France for years and your room makes you sad than yelling about how expensive your room is.
• Especially outside Paris they're not making it up: many people really don't speak English. Speaking loudly won't help. Google Translate and other translation apps are your friend.
• In schools there are a lot of vacations: two weeks every 6-8 weeks. There are also subsidized camps for the kids during this time. Wednesday afternoon is off for schoolkids. There's a very strong chance your kids teacher won't speak English but an equally strong chance they'll work with your child to bring them up to speed. Your kid will soon speak French much, much better than you.
• Dogs are welcome almost everywhere, including restaurants, and people smoke outside. There's a fine chance your waiter will take a break and have a smoke. If you can't deal with that you're going to have problems here.
• You might be asked about the latest political happenings in the US (ex: as I write this The Donald is raising eyebrows). Answer honestly; the French are all over the place politically.
• Don't rush meals, or anything else necessarily, but especially meals.