I don't think it has to do with class representation or win-rate; I think a good meta has more to do with the following two items: 1) Player preferences are respected, and 2) In-game decisions matter.

1) Player preferences matter: I don't think your average Pirate Mage player expects a 54% win-rate. Nor should a "utopian meta" give them that. But what players DO want is to feel like their preference for Mage decks aren't a death sentence in the face of "Fuck You Warrior" at the top of this imaginary meta. There has to be some give and take negotiation between not only the players and devs, but also between players and other players as to where those lines of preference respect are drawn. "I like Mage decks" and "I like aggro decks" seem like fair places for Hearthstone to try and hit preference - "I like aggressive Mage burn decks" might be a little too specific to hit every single meta (while still providing variance in archetype from one set to another). A good meta gives players both fresh and viable ways to experience their preference, within reason.

2) In-game decisions matter. There's a lot of creative ways this can blow up. Evolve is the hot button right now, where it feels bad because there's a chance variance goes against you and the game was a non-game because the bunnies and 8 mana rushers turned into chads. It can feel like "it didn't matter" because your opponent curved out 1-2-3-4 and you didn't. It can feel like "it didn't matter" because your opponent was playing a "non-interactive" combo deck. This is really hard for any card game dev to get right, because players are often wrong about "well nothing I did mattered" when they don't-know-what-they-don't-know about decisions they could have made, or the importance of deck building and mulligan priority in certain matchups. This is where design has the hardest challenge of delivering set after set - creating archetypes powerful enough to keep metas stable (like a hyper aggro face deck) that also feels skill testing and rewarding for better players. The best metas that we've had, at the very least, didn't have decks (or many) with too many "Whelp, Undertaker on 1, GG" type moments.

If you hit both of these points, you've got as good a meta that can be achieved in a card game. If players feel like they can play in their preferences with decisions that matter, you've made a knockout set.