When I was in Paris, I was lucky enough to meet up with a cousin who lived in the city. They chose Le Fumoir as a meeting point for a couple of reasons. Firstly, its location is very central and is in walking distance of nearly all major attractions. Secondly, the bar had a long history and reputation as being a fumoir (smoking place), but due to law changes, smoking inside restaurants had been banned. This means that is has the old ambiance and feel to it, but is clean and inviting for non smokers. And lastly, the food there is consistently of high quality.
The lighting is fairly dim, with candles on the tables. Settings are with linens and fine silverware. I do not know of a dress code, but it is not entirely casual. Dress is typically more business or tasteful. Reservation is recommended.
Favorite Dish: All the dishes that we had were fantastic. The steak and garnishes were savory. The fish was extremely tender and the desserts were decedent. I'd definitely go there again.
Le Fumoir is situated opposite of the backside of the Louvre (not on the Jardin des Tuleries side but on the other end). I had lunch there only once - but i was impressed by the quality of the food. Service is excellent too. It's a bit expensive though, but hey ... you live only once - at least that's my personal conviction.