For me, it’s just countries I have never drawn, rarely draw or don’t draw as often.
Singapore is so-so in terms of rareness I guess, although we have definitely become a ‘common country’ to the Malaysians now because their postal restrictions only allow them to send postcards to us. 
I have had Postcrossers telling me they are happy to receive postcards from my country and it was their first. 
But rare or not, I just love sending my postcards anywhere. 
I’d say Greece is very rare. I received from it once in my early days and knew it would never happen again!
I’d say anything except the top 15-20 countries (based on number of sent).
I’ve never received from Italy or South Korea even though they should be “medium” 
Never sent to either of them, too.
I used to receive from Romania “a lot” (like, 4 cards 
) and only recently realised it’s considered rare.
Covid restrictions gave me a lot of Malaysia and Indonesia that I wouldn’t otherwise get
No. I don’t think so
Yes
I am finding that I am frequently sending someone their first card from Turkey. I think it was more common at some point - I had received two or three from Turkey in the past - but seems it has been more unusual recently.
I would probably define rarity by frequency. If I get 1 card per year or fewer from a particular country, that’s pretty rare.
I would factor in number of active members and frequency of sent postcards to determine rarity. Apart from that, efficiency of a country’s postal service would also be a factor.
I think there was more active members in Turkey before. Probably many people had to leave because of higher postage prices.
updated answer.
Any card from any country or territory in Africa, Oceania, South America, Asia (excluding China, Russia, and Taiwan), the Caribbean.
Any European card from Jersey, Guernsey, Isle of Man, Transnistria (it would technically be a Moldova card), Kosovo, Andorra, Liechtenstein, San Marino, Vatican City.
You’re right regarding members acitivity and how the post works. But more important thing is, I think, the population. If in your country there are 1342 950 000 inhabitants but in my- 1893 000…
Countries with numbers of postcards sent in the 5 digits or less.
Since then I got two other cards from Ukraine, no complains, but not sure if Ukraine is turning now to the opposite of rare?
Maybe just luck.
Being from a rare country I feel like I have an unfair advantage, in that even if I send a mediocre /boring card (which I definitely try not to), people are still surprised and happy to receive from me.
Anyone else from a “rare” country share this feeling?
I know how you feel.
I always try to send the best card I have because I want to send a positive image from the country I am in.
I love being in a rare-ish country, althought there are limited amounts of postcards availiable (touristy ones) the majority of the time I’m the first person they have had contact with from my country so I feel like a ambassador. Sometimes I get a reply that they have visited the island so it makes me even happier.
We should probably do one day a rare country swap or something.
I actually get a lot of people saying they visited (Malta being a popular Summer holiday island). Once someone even told me they got married here 
Malta is beautiful, I visited for the day on a cruise and would love to go back. We went to Marsaxlokk and Mdina 
I am from Sri Lanka 
. Does anyone consider it as a rare country to receive a card?
I suppose yes.