You could make a good deck in MTG out of all common cards too, but just like in this game, if you wanted a deck that was highly competitive, it was going to take a significant amount of rare cards mixed in with those commons.
My worries about the game are this: There really aren't that many cards. And some of those cards are largely non-competitive unless you are working a weird niche that itself isn't that effective. Given the limited selection of cards, the massive playerbase that Blizzard will inevitably pull in, and the online nature of that competition, I'm worried that the meta good decks will become pretty stale. Even people without all cards available are pretty likely to mimic those decks with stopgaps in their deck construction, since it's going to work. Now the meta growing stale was a concern with MTG as well, sometimes it got pretty bad, but with thousands upon thousands of cards to choose from, along with decks being 60 cards instead of 30, with creatures being sometimes an entirely optional component, and mana resources being something you had to build into the deck instead of being paced, and the ability to mix and match colors/classes within the same deck: it seemed like you had a lot more room to think, "I have no idea what deck I'm going to run into next." I won a couple dippy local tournaments with a deck that was all commons and in fact wasn't very good. It was just ran a mind game with unexpected pacing that the standard good decks weren't prepared to handle when played "properly."