A small observation: So far, I've noticed that many of the "restaurants" mentioned above, while they are places where you can eat, aren't really restaurants in the traditional sense.
Quite a few are technically closer to the fast-food chain category, like those in the Louvre Museum that serve pre-cooked, industrially produced food delivered to various locations such as airports and museums. One of the basic rules: always avoid "restaurants" in museums or other tourist attractions.
They fall into the same category as highway rest stop restaurants. Similarly, pizzerias are not considered French restaurants either.
The one mentioned by davebarnes is a hotel-bar-PMU-tabac, one of those places frequented mainly by locals who stay for hours, come to have a drink, buy cigarettes, and bet on horse races, 99 times out of 100, it's a red flag.
B wary of restaurants that offer onion soup. The French haven't cooked onion soup at home for a long time (it was a dish for poor people made with inexpensive ingredients ). It can be good and well-prepared in more modern versions, but it's primarily a dish for foreign tourists.
That being said, even when following a few basic rules, it's very common to end up in a terrible place. I know loads of them.