So as promised, I found acceptance to be as following overseas (and not mentioning hotels - that's just a given):
HONG KONG
Accepted in major chains like Bonjour, and at the Ngong Ping chairlift. Accepted in almost all of the market stalls/shops in Stanley, but most of my shopping in HK was at the markets, so mostly just cash... Not accepted in Aji Ichiban Japanese candy store chain, or in H&M.
CHINA
Only visited Shenzhen this time - not really accepted at all (though most of the shopping is market stalls).
SINGAPORE
Accepted widely - including seemingly all taxis that accepted any cards, SkyPark, restaurants, and all stores that I visited at Changi airport.
Be aware that with Amex, many credit card terminals in Singapore will prompt the clerk for a "4DBC" before processing the transaction, and this seems to confuse many staff. The 4DBC (4-digit bank code) is the four digit verification number on the front of the card, not the card's PIN. If their English is poor, just point to the 4 digits on the front, and it'll work just fine!
UNITED KINGDOM
Widely accepted, particularly in London. Accepted at all tube stations, but if you don't have a chip with a PIN, just head to the window and the clerk can swipe your card instead. Note that not all black cabs accept cards - check before entering. Reasonable acceptance in Scotland too, and seemlingly a little less in Wales (but you can still look for the logos).
FRANCE
Reasonable acceptance. Not accepted for Metro/RER tickets, at the post office, and at some smaller shops/restaurants. However, it was accepted much more widely than some others seem to suggest. I didn't have any issues with my non-chipped Amex card being swiped and providing a signature. One store had to key-enter the number into the terminal because the magstripe reader was broken, but this was done without issue.
ITALY
Reasonable acceptance, but always look for the logos. Not accepted for metro tickets in Milan, or water taxis (and gondolas) in Venice. Accepted widely in Venetian stores.
AUSTRIA
Very widely accepted in Vienna. Same situation for taxis as for the other cities mentioned. Moderate acceptance in Salzburg. Less welcome in Mondsee.
CZECH REPUBLIC
Was only here for the day. Found it to be accepted in some souvenir stores, so just walk along and look for the logos. Note that the souvenirs are usually identical from shop to shop, so you're not really missing out by skipping those that don't accept it (unless they're offering something unique of course). Found card minimums to be from 100-150CZK.
GERMANY
Didn't have to use MasterCard once in Frankfurt or Munich.
CANADA
Very poor acceptance (the worst I've ever seen - odd considering it shares a border with America)!!! Accepted in cabs that take plastic (in Quebec City, Montreal and Vancouver anyway), though most non-chain stores won't touch it, and even some chains and major businesses still refuse. Notable declines include the Snow Village (ice hotel) in Montreal, Vancouver metro, and the Montreal metro (where it seems that only Canadian cards are accepted - both my AUS MasterCards were declined despite being valid). Major accepter: Air Canada (including on board purchases). Some Canadian platinum Amex cards give priority security clearance in certain AC airports.
USA
NYC: accepted *everywhere* except for one souvenir store that were still displaying the logos (the ruined my 7 day streak of no MasterCard usage! haha)... They generally look at you strangely if you ask if they take Amex, as though it was like asking them if they accept cash. Instead, I ended up asking if they "take credit" instead, and only when there were no card logos on display.
Chicago: Accepted practically everywhere, except for photos at the John Hancock observatory. Problems using it in one taxi, but only because the entire Amex network appeared to be down (tried 2 cards from different issuers), and after about a minute each, both cards said "no response". My PayPass Mastercard did work. No problems using Amex in a cab the next day - the network was obviously restored.
San Fran: Seemingly universal acceptance (restaurants, taxis (do check for card logos or ask first though - not all accept plastic), the Super Shuttle from SFO, etc etc)
Vegas: Again, wide acceptance. Taxis charge $3 extra for any card transaction, but again, not all cabs accept. When arriving as LAS and joining the cab line, tell the taxi marshall you want to pay with credit, and he will have you stand at a particular stand to ensure you get a suitable taxi.
LA: All Yellow Cabs take plastic, but some drivers don't like to take them... one said "no chance cash?" when I tried to pay. I told him I only had foreign currency, and would obviously tip him on the credit card, and he removed the cover off the hidden credit card machine quite happily. Wide acceptance, and the "offical card" of Universal Studios Hollywood, so it's accepted everywhere inside (as well as for tickets).
General USA observations: we didn't encounter any credit card minimums or surcharges. Major chains that accept include McDonald's, Walgreens, Starbucks (never found one anywhere in the world that wouldn't accept Amex for just one coffee), and IHOP. Some stores asked for ID - no issues when presenting an Aus drivers licence (I wasn't going to carry my passport around just to use a credit card)! I started the trip in the USA looking for traditional EFTPOS-type machines, and often didn't see any. Many instead have card readers integrated into their register systems (such as in the screen), which are generally completly safe and bank-certified, provided they only swipe the card through one device.
NEW ZEALAND
Have only visited Auckland since having an Amex. Widely accepted, except for the Skytower and their Skywalk/Skyjump/Skydeck activities. Was accepted in the gift shop on the lower floor though! If catching the "Airbus Express" (airport-city bus), Amex can be used to pre-pay online (if you have a printer), or at the kiosk at the international terminal. Only cash is accepted on board though if you haven't already purchased a ticket beforehand.
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If anyone is travelling to any of these countries and would like a more detail, just let me know and I'll update my wiki (
American Express Acceptance Wiki).