I agree that nobody should really expect to just occasionally play Hearthstone and pay nothing, it's not really fun that way because you require some staple cards that is unrealistic to save for with credits (gold)... I'd only really start playing this game if you prepare (/ are open to the likely possibility when you try it and you like it), to invest a great amount of time and energy in it, and also money for the single player adventures to earn some vital cards otherwise unavailable to collect or make yourself, and maybe some packs to get started. But I don't feel like I have to keep paying money indefinitely for packs like a bottomless pit - I play in Arena to compete for them so that I gradually but slowly get more cards, although it is hard to earn something extra there. Also expansions will make you want to keep up.
Basic mechanics are not hard to understand, but the game can get pretty damn complex to play well. You don't have to be a supergenius, but it will cost time and regular play, not something for when you have 10 minutes to spare every now and again.
I hardly ever play games that really cost something, and I refuse to pay for freemium phone games from say Supercell - and you can still play them for free if you really want to, also they aren't really so brilliant that they are really worth paying good money for.
But Hearthstone is a real exception if you ask me.
Don't compare it to freemium phone games even if you can play it on phone or tablet, it is more comparable to Magic the Gathering or other card collecting games (though I have no experience with other card collecting games, but this comparison is more reasonable), which also are not free and I never heard people complaining about microtransactions from having to buy actual physical collecting cards... it is not realistic to expect all of it to be free, with phone games also the money has to come from somewhere... but many of them aren't really so high standard that it is deserved of the insane amounts of money that some people put in over time.
Giving people the expectation that it is free is a dick move ^ - I'm not adding you, and I can't imagine someone losing more than you.
HS costs money to play at least somewhat competitively and to really learn, and it simply is a competitive game unless you solely play your little sister who is equally broke. But it's worth it.
My favorite card is the Rogue card Crackhead.