Some good advice here. I could add:
1. The priorite a droite rule was universal, but was abolished a long time ago. Main roads have yellow lozenges along the route, indicating that the main road has priority (same as here). This seems to be adhered to pretty well; the only places anyone has pulled out in front of me from the right was in small villages where the priorities are not clear anyway, and what is obvious to us is the opposite of what the French do. But if you keep your speed sensible and are cautious, it's not a problem.
2. The autoroute tolls are well worth it if you want to get somewhere quickly. If you are going from Calais to Marseilles, the cost can mount up, but beautifully-engineered roads, light traffic and an 80 mph limit in dry weather make up for it. Even off the motorways, traffic is less than in the UK (same population with 3x the land area). I find driving in France a pleasure.
3. Signposting on the autoroutes is similar to the UK, and of high quality. On the D roads, it can be a bit confusing, although once you get used to it it's OK. Approaching a major crossroads in a town centre, for example - say you are heading for Paris which is a left turn. In the UK the signpost would be on the left exit, pointing away up the road. In France, it might easily be on the right exit (facing you) and pointing left 'across' the junction. It's quirky, but getting less frequent as France gets subsumed into European standards and norms. It's a shame, as French quirkiness is excellent. I'm also a fan of older Citroens, for the same reason.
4. Following road numbers is often easier than following destinations on signposts, but beware. I once pulled off a autoroute onto a slip road, looking for the D440 (for example). At the roundabout at the top, there were four exits - all labelled D440. I had a caravan on the back of the P38 and it caused much confusion, donuts and laughter before we finally guessed the right one.
5. Away from the big centres, there is much less of a 24-hour culture than we have in the UK. Shops will often close for 2 hours over lunch, especially in the South, and Monday is often like Sunday used to be over here - shops, banks, everything closed.
6. Bring a couple of different credit and debit cards if possible. Often in the country there will be an automated filling station with no attendant. Some of these do not accept UK bank cards, which could leave you stuck if you weren't aware. I can't remember precisely what the problem was (chip and pin?), but I know that with a Visa credit and a Visa debit card I was always OK. I believe this situation is improving as well.
7. As has been said above many times, drive defensively. The French are not bad drivers as a whole, and it is different rather than dangerous, but it's as well to remember that their road casualty rate is almost double the UK's.
I really love France, and I find the French helpful and friendly. This is possibly because even though my spoken French is average to poor, I always speak it wherever possible. The French are very proud of their language and culture, and really appreciate it if you make an effort. I'm usually in places where English isn't spoken anyway, and we often end up using sign language, but I have never failed to get what I want, and had a lot of fun with people on the way. People who think that 'foreigners understand English better if you shout' will probably find the French rude and ignorant. Quite right too.