So far, Hearthstone: Heroes of Warcraft has shown Blizzard's impeccable talent for game balance. Despite the occasional need for adjustments, the studio has smartly found the proper balance of letting the meta-game evolve using the tools provided. All that careful balance is going to see a huge shift with the upcoming release of Curse of Naxxramas, and not necessarily for the better.
Hearthstone has cautiously avoided the "pay-to-win" label. By randomizing powerful Legendary cards and choosing not to include any kind of trade or bartering system, the underlying economy has remained essentially sound. Pro players have shown over and over again how, with enough skill, you can reach Legendary rank without spending a dime. Crafting Legendary cards is so outrageously expensive that it's more a consolation for junk cards than a goal in itself. It's this magic combination of randomization and a solid economy that has made it so inviting.
Much as I'm looking forward to Naxxramus, I'm afraid of the inevitable dramatic meta-game shift over a very short period of time. More importantly, I'm concerned that Blizzard is undercutting those two elements that make the balance work so well. The powerful Naxx cards are prizes, not random Legendary drops. On top of that, you can practically purchase them outright. Yes, the actual method is to buy single-player campaign stages and then earn them, but the net effect appears to be the same: you'll be putting money into a piece of content that guarantees you access to certain powerful cards.
I should note, of course, that you can purchase the Naxxramas cards with in-game gold, but fans who have not already saved up are more likely to buy with actual money. Averaging 50 gold per day--the midpoint between the two most common tiers of daily quests--it would take two weeks to earn enough gold to buy just one Naxxramas pack. The bundled discount isn't available for paying with gold. Getting all five would take months, well past the time that other players will have incorporated it into their own strategies.
Obviously Blizzard wants to make money on this expansion, and it's hard to blame them. Hearthstone is a huge hit, and I think gamers owe it to themselves and the studio to toss some money into the studio's coffers if they've been enjoying it for any significant amount of time. Still, tying such a competitive edge to sales feels contrary to the groundwork and goodwill laid so far.
I'm sure, also, that shaking up the meta-game is part of the point. The current meta-game is something of an escalation match, as high-level players develop new strategies designed to undermine whatever the current dominant class type is. Miracle Rogue trumps the Hunter's hounds, then Frost Mage freezes Miracle Rogue's removal options, and so on. By throwing so much new into the mix, Blizzard can disrupt this trend and force a new several-week or even months-long period in which players figure out new strategies.
Even at this, though, Naxx may be creating a greater imbalance. Dominant classes like Hunter and Rogue are getting cards that appear to augment their existing strengths. (The Rogue ability to return a friendly minion is a staple of the Miracle Rogue, and it comes with a 5/5 creature to boot.) Meanwhile, classes that consistently get low marks in the power rankings are getting Naxx cards that can easily be made functionally useless, or even a detriment. The Priest and Paladin's both buff another minion upon death, meaning a player will simply have to wipe it out last. The Mage's secret, which gives two copies of a defeated minion, could easily be used to clog the hand by giving her two junk cards.
I have enough faith in Blizzard to think that it considered this, and has already play-tested strong strategies using under-powered classes. Plus, it's comforting knowing that the studio has metrics in place and is willing to adjust the game as necessary when unforeseen problems emerge. My own reservations certainly won't stop me from getting Naxx and playing alongside everyone else, while keeping an eye on how the community shifts its strategies in response. My trepidation isn't about my own impending losses, but rather the notion that someone who doesn't spend the money won't stand much of a chance.