Driving, parking and scooters!
The French have a bit of a berserker, rebellious mentality when it comes to driving. They often violate traffic laws, and usually flagrantly. You don't even have to be driving in France for this to be an issue. Watch carefully for rogue cars when crossing streets as well.
The most annoying aspect is the scooter in France, whose drivers usually take advantage of their ability to go anywhere. You may have to dodge them on sidewalks, they often head the wrong way down streets and, besides all that, they are obnoxiously noisy. And since fuel is pricey in France, they are everywhere. And they park wherever they want.
Speaking of which, the French will park anywhere they find a hole that will (or might) fit, even if it means the sidewalk, double parking, blocking traffic, blocking the handicapped ramps on the sidewalks, tapping the bumpers of a few cars to park, and so on.
Smoking
Everyone knows the French love their cigarettes. This means that it will be hard to avoid smoke while visiting France. This is another situation that has improved, quite dramatically in a very short period of time.
It used to be that there was no such thing as a non-smoking area of France. Now, most airports and train stations are smoke-free, as ar many malls. Some restaurants, especially in touristy areas, are starting to offer non-smoking sections. There are non-smoking train cars.
The transition hasn't been too natural, though. Usually non-smoking sections are behind the smoking ones, which seems a little pointless. You will usually find ashtrays in "non-smoking" buildings, and they are usually filled with butts. Many French people boldly disregard the rules.
If you are trying to avoid smoke, I would suggest visiting when the weather is nice and frequenting sidewalk cafes. Stick to the tourist areas, where you are more likely to find non-smoking spots. If you plan to stay at a hotel, ask them if they have non-smoking rooms (and if they are on non-smoking floors, or else you may be right next to smokers). Your best bet for hotels with true non-smoking rooms (as in, don't smell like the last smokers who stayed there) is to stay in big chain hotels.
If you are a smoker and happen to think this is a positive aspect of a French trip, be sure to check out my feature Smoking in France: One Place Where Lighting Up Is Still Acceptible.
Unpredictable hours
It can be a bit of a challenge getting into the rythym of the French schedule, in which shops often close for long lunches, and restaurants usually close between meals. Get tips for coping with my article The French Schedule: Surrender to the France Timetable.
I know this isn't the most flattering of descriptions of France, but these cons are far outweighed by the pros of visiting France. This article is designed to prepare you for what to expect. But, like the French, just roll with it. You can't enjoy the "joie de vivre" if you obsess over the negative.

