I'm going to try to be as fair as possible.
Pros:
1. Addictive game play. With hundreds of unique cards to toy around with, you can never get bored of this game. I can only play this game 2 days a week. On those 2 days, 90% of my time is spent playing this game. No matter how frustrating it can be sometimes (I'll talk about that in cons), it still manages to keep me addicted.
2. Packs & Gems. With the ads giving you gems and how cheap packs can be, it's a really rewarding system that makes the game feel f2p. I've noticed that the luck of packs has increased recently, with 80% of premium packs I open containing at least 1 super-rare or better. This, plus the powerful event cards everyone can get, makes the game enjoyable for everyone.
3. Balance. This was already mentioned by @daalnnii but I want to mention it again because the balance in this game truly is great. There is an exception, however...(will talk about in cons)
Cons:
1. Deck limit. The deck limit recently has decreased form 100 to 90. Yes, strategy decks were added, but I would just consider them options/recommendations and not actual decks. I usually make 5-6 decks per hero: Control, Aggro, Major tribe #1 (Ex: Pea deck), Major tribe #2, Something unique to the hero, and maybe something else focusing on a specific ability (Self-Hurt, Anti-Hero, Armor, ect.). Now, the limit is 4 per hero, with 5 extra per side. It feels so limiting, at least IMO.
2. Sneaky. Every class is so balanced, but this one...no matter what deck, play style, hero, or class I use, Sneaky wins against me roughly 75% of the time. Why? Well, it has no definitive weakness. Tell me what play style, hero, or class counters Sneaky. That's right, virtually nothing. Anti-Hero + Smoke Bomb is such a cheap combo, and due to Stealthy Imp only being an Uncommon and Smoke Bomb only being a Common, EVERY Sneaky player has access to it. Oh yeah, and Surprise Gargantuar. Smoosh-Shroom is a 5/4 5-cost Common. Surprise Gargnatuar is a 5/6 5-cost Common with "Gravestone. When revealed: Move this zombie.", plus it's a gargantuar. WHAT?! Also, Imp decks. No card disadvantage because of Commander, and no minion disadvantage because of Toxic Waste Imp. Very balanced. Also, Deadly in general. A re-usable free Locust Swarm whenever a zombie with this trait hits a plant. Perfectly balanced. And now that Fire Rooster is out, not even mushrooms can defeat it. Please, EA...if there were a Weed Spray for plants, that would make Sneaky balanced, because so many of its zombies have 2 strength or less.
Phew...long rant. Anyway, that's all.