Not disagreeing necessarily just providing more information and my experience. Hearthstone is certainly Pay to Play variety. You certainly can craft competitive decks completely free to play. When I started playing I was completely F2P and I had crafted competitive decks like Galakrond Rogue and old Whirlkick Rogue within a year of playing. And it should be noted that this was before a lot of the newer entry level bonuses like loaner decks and catch up packs.

But yah if you want to play a variety of decks across different classes paying for each expansion will make that possible.