These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
That One Boss: A whole gauntlet of them near the end, culminating with Neo Exdeath.
Scrappy Mechanic: The battle system. Just swipe your finger/stylus over your characters like mad, and you have a system. That's it.
Having to pay for new areas and new characters is a dick move, but excusable since they are optional extras. What isn't is that the gold hourglasses you need to revive the party cost money. Yes, when your party is getting pummeled, your only options are to wait three minutes of real time as they revive automatically, or pay money to revive them instantly. You have to pay more money to revive your party members.
They Changed It, Now It Sucks: The main accusations that reviewers like IGN and Game Informer make whenever the game and its battle system is brought up.
They Just Didn't Care: The whole game just screams "zero effort for a quick buck". The entire gameplay just consists of swiping the characters repeatedly with no strategy whatsoever, and the game is built in such a way that tries to get you to pay money to bring fallen characters back. The sprites save for some "premium heroes" and the soundtrack is all recycled from other games.
They Wasted a Perfectly Good Plot: Based on premise alone the game could have been great, and the sprite work is awesome and shows there was real effort put in. The result has not only been underwhelming, but completely hated by fans and reviewers alike.