It is a great game but it has a few flaws.
-Cooldown times/energy
With the key system it takes forever to get something. The dungeon keys are the best, as you can build up quite a few if you want to grind. The gauntlet key time should be shortened because a full day is too much. The arena system is very grindy and takes forever to get a key or even recharge your heroes.
-The Pay To Winners
I get every company needs to make money but really, there are so many people who pay tons of money to get OP cards like hammer of God. That one card has level 6 players doing 700 damage and splash. I COULD pay, but the gems are so overpriced. It is nice you can get a good amount of gems without buying, but even at fame 9 the campaign is very difficult for characters that are level 15 and destroy people who dont pay (like me) in the arena.
I hope kongregate fixes these problems as they are a great developer and I really love this game.
Edit
Oh wait I forgot gold is almost useless as you can only buy booster cards and battle cards with premium currency and gold packs, the only card pack with good elemental cards, cost more than $5. I know I could get a gem dropper or a character pack for cheaper, but the dropper means you need to log in every single day for a month or you lose your gems. And some character packs are trash because the said pack contains not enough materials to get the card good for even my fame level. Other character packs are specialized to a hero and not a class, but you might not always use that hero.
Edit 2
Now the character packs are bad because they replaced 700 ($15 worth of gems) with a second rare and a five dollar discount. Is a rare card worth 10 dollars? No. Im level 12 now with level 19 heroes because it is hard to level up heroes with the current dungeon key system and now I dont have enough gold to level up my new hero and his powers so he is very easy for enemies to kill. Aso, to do his quest I have to use a gauntlet key and I only get one every 24 hours. The wishng well and character packs are cheaper, but are really not worth it. There is still a pay to win problem.