Hearthstone is a trading card game released by Blizzard in March 2014. It's available on PC, Mac, iOS, Android, and more platforms because it is built on top of Unity. Hearthstone had 25 million registered players in January 2015. If you come from a traditional pay-to-play sector of the game industry, you could say that Hearthstone has twice more players than World of Warcraft, and be impressed. But if you come from the free-to-play sector of the game industry, you know this number means nothing: the game is free, so players will eventually try it out. In the mobile F2P market, nobody reports "registered players": what matters is the number of monthly users, and the average revenue per user. In fact, if we look at Hearthstone as a free-to-play mobile game, it is an outlier in several ways. And it's not necessarily a bad thing.
First, the business model. Players can earn coins, a soft currency in the game, by completing daily quests. One new quest becomes available per day, each quest takes 1-2 hours to complete, and grants 40-60 coins. Players can also get coins by winning games against other players. The amount is much smaller though: 10 coins every 3 wins (a match takes 10-15 minutes). Each pack of five cards cost 100 coins or $1-$1.5 if bought with real money. Cards can also be crafted from dust, another in-game currency. Dust can be acquired (nearly) only from cards bought with real money. So the game is free to play, but with 5 new cards roughly every 2 days, non-payers will find it excruciatingly slow to progress compared to other free-to-play games.
Second, mudflation. Some cards from the Gnome vs Goblin expansion set clearly outclass cards from the base set. For example, the Murloc Raider from the base set, has 2 attack, 1 HP, and costs 1 mana. The Clockwork Gnome from the expansion has the same stats, AND gives a 1-cost utility card to its owner when killed. I admit that this kind of outclassing also happened between the free cards and paid cards from the base set. For example, the River Crocolisk from the free base set is outclassed by the Amani Berserker and the Bloodsail Raider (both paid/crafted cards). So balance-wise, it's as if the set of paid cards was the first expansion, and Gnome vs Goblin cards the second. Either way, non-payers can't compete against players who purchase (expansion) cards.
Last, the daily quests. Blizzard first came up with daily quests in World of Warcraft, 6-7 years ago, to give people things to do while waiting for the next content patch. They had removed the cap of daily quests completable per day, and realized that "that really leads to burn out". Mobile free-to-play games started using daily quests as a way to retain players, 3-4 years ago. And now, Blizzard is back to using daily quests to retain players. But this time, they cap the quests to 3. Compare this to the dozens of achievements in Clash of Clans, where there's always something to look ahead for. Players could play hours at a time to push for Clash of Clans achievements. But in Hearthstone, quests run out quickly, so there is little reason for players to stay longer than necessary (except maybe leveling up or winning, but the rewards are so much lower than daily quests that they don't seem worth it). I also suspect that players eventually get tired of doing the same quests again and again. Cleaning up Naxxramas brings a good change of pace, and has very interesting mechanics, but each aisle cost 700 coins, so it only happens once every 2 weeks. Too bad ... So daily quests: somewhat good for day-to-day retention, but probably bad for session duration.