“The beauty is in the eye of the beholder”, the difference is simply that people are different.
What comes to the Postcrossing’s amount of favourites, it is not a sure system to determinate whereas the card is lovely, ugly or boring. If we want a public opinion we should have as many people place their answer as possible (eg. show a card to 100 people and ask them to say if they think it is boring, ugly or lovely) Then we would have some kind of a general opinion. If we show 100 different cards to these same 100 people, we could estimate what this specific group generally favours about the cards, but we will still not know what individuals prefer.
However, in Postcrossing there is no jury of 100 people deciding on each card if they like it or not. You card is seen by a random number of people, depending on how active you are on forum and main site (aka. how often your profile appears easily “clickable” and how much interest people have on checking your wall), if the card has been specifically mentioned and linked somewhere (eg. a post on forum), and if the sender or receiver is ambassador, supporter, in top10 of their country, from a country with a low member count, mentioned on blog… These all affects the chance of other members to end up browsing the member’s postcard wall which then again increase the chance of cards being favoured. Also, of course sometimes people don’t want to have favourites on their wall, or they favourite only cards they receive, or cards they want to receive, or only one or few cards of each theme they like. So the amount of faves essentially tells nothing about the lovely-ugly-boring rate of the card.
Postcrossing has a huge postcard gallery with several hundreds new uploads each hour, so it is highly possible that nobody else than the receiver and the sender will ever even see the card.
And when it comes to my personal favourites, I like to not announce what I specifically find lovely as often I end up finding something else that I also find lovely and my opinions change along the time. But I do admit that I am especially fond of horse cards, and the cards with glossy look feels uncomfortable on my fingers (I admit, twice I have favourited card purely because the quality was so good it felt nice on fingers). However, I give faves also just because I liked the message, but I only favourite the cards I have received myself. So for this example about Japanese shrines, yes if I received one and liked the message or/and the card itself, I would favourite it. But because I only favourite from the cards I have received, I wouldn’t go around browsing and favouriting shrine cards other have received.
Also, I dislike the certain popular card series with facts because I found those facts very basic and also incorrect in some cases.