Can someone give me the rundown on where my American credit cards will not work because of the lack of the microchip? Mine has been rejected at pay-at-the-pump gas stations, but I understand that it also may not work on some trains/metros etc. Can anybody clarify?

For the most part, they should work most everywhere as the terminals are programmed to recognize the magnetic stripes as well as the chips on the portable machines they use to process cc's, they work for the most part on the metro ticket machines; the not working at pay at the pump petrol stations is the exception not necessarily the rule.

Posts have said you must use a French CC in the automated kiosks for RER tickets at CDG. But other train tickets (SNCF) can be bought from the machines with US CC's.
ATMs will work, and it will work in any retail establishment for purchases, hotels, restaurants, etc. (just those pesky gas pumps - but I'd think you can go inside and pay with your US CC.)

When hypermarkets are closed (like on Sundays or at night), the ONLY way of buying fuel from their stations is with a working chip-embedded card.

So those of us with steam-age cards have to try and find a steam-age petrol station that's open. And pay the substantial premium you have to fork out for patronising BP or Total.

Point of sale terminals operated by human beings normally accept both modern microchipped cards and old-fashioned cards with magnetic stripes. ATMs will accept both as well. Some other types of self-service machines, notably the ticket machines in the RER and Métro, will accept only cards with chips.

In self-service machines, you'll generally need a PIN, no matter what kind of card you have. With POS terminals operated by people, you can just sign for a purchase if your card is the old magnetic-stripe kind.

Be wary of pickpockets, as they love the old-fashioned cards, since they can steal them and charges things on them just by forging a signature, without the need to know a PIN.

I've always been able to use my old fashioned magnetic stripe cc in the metro vending machines.

I think it is specific to the CDG - RER machines.