Here are some that come to mind for me.

"New York is just a giant crater."

This idea mostly comes from the Fallout Bible; a series of documents from the early 2000s designed to provide background on the world of Fallout. It is considered non-canon and the quote that references New York states the following.

"June 2065 Due to enormous demands for electricity in the summer of 2065, a nuclear reactor in New York City almost goes critical. The near meltdown brings into effect power rationing, and the term 'Hot Summer' is used to refer to the New York incident."

There is no mention of New York being destroyed.

In Fallout 4 the Civil Alert System Broadcast says New York was hit by a nuclear attack but also mentions other places that we have seen are still habitable in-game.

This morning, Saturday, October 23rd, authorities detected that a nuclear attack had commenced against the United States. The following cities affected include, but are not limited to: Washington, DC; New York; Philadelphia; and Boston."

Fallout 76 Wastelanders DLC's has Johnny Weston stating the following once again suggesting New York was habitable after the bombs fell.

I don't like talking about it, but I know you'll keep it secret. My dad was military. I saw army bases all over the country as a kid. When the bombs dropped, I was a bum. A failed actor, living on the streets in New York City. My parents never approved of acting for all the reasons you'd expect from a family with strong military tradition. Anyway, that's the story I'm telling you. You can decide for yourself if it's true.

On top of all of this in a documentary by NoClip a developer states New York was originally going to be the setting for Fallout 4.

"The US in the only country that remains."

In Fallout 3's The Pitt DLC Ashur says the following in his speech

"Citizens of The Pitt, workers of Downtown, traders of Uptown, and all the fierce souls who do what must be done! I bring you good news! We stand at the dawn of a new golden age. Where others merely survive, we thrive! Our industry is the envy of the Commonwealth! Our safety is the envy of the Capital Wastes! Our might is the envy of Ronto!..."

Due to where The Pitt is it's speculated that Toronto, a city in Canada, may be what he's referring to.

Outside Vault 101 there are letters in mailboxes saying there are Vault-Tec brand vaults in Canada

For a full list of Vault-Tec facilities with available accommodations, in exciting locales such as Oklahoma and newly-annexed Canada, contact your local Vault-Tec representative!

In an interview with Emil Pagliarulo and in Fallout 3's game guide it's confirmed that Allistair Tenpenny is from the UK.

A terminal entry from Tenpenny Tower says Colin Moriarty is also from some other country.

I hear Moriarty even has his own place now. Guess that shouldn't surprise me. That guy had been playing the angles since the day he ended up in this country, as a kid

There are references to other countries in non-canon materials like the Fallout Bible and Fallout Extreme that would suggest some areas outside of the US are habitable as well.

"Functional cars and trucks don't exist in the world of Fallout"

The Highwayman is usable in Fallout 2 and there are a couple references to vehicles being used but not shown in-game like the NCR's trucks in Camp McCarran or Steam Trucks being used by Super Mutants in the original Fallout.

I imagine this likely an engine limitation or a decision based partially around how it would effect the map's design.

Edit: I didn't mean to sound pretentious by using the term casual fans. More so newcomers or people who have only played one or two games in the series.