No one will object to mana cheating being problematic. The real source of the degeneracy are all the cards that allow players to refill their hands for free. You know the ones. If a player can combo off and dump their hand and be done with it, fine. But the fact that they can refill their hand and find more gas is what enables some decks to the oppressive. I’m all for card synergy, but free/cheap card draw has been a problem since wizards printed Ancestral Recall. If one thing should be fixed, it’s this.
Here are the major offenders in my view:
Gear Shift
Secret Passage
Cutting Class
Spectral Sight
Spread the Word