I would have three rules.
- If a card dominates a format, where every deck is about the card, or has to be an aggressive counter to the card.
- If a card is in every deck, regardless of what the deck is about.
- The card makes games really boring, or makes you do a lot of work, disproportionate to its effect.
The obvious counter argument: "If everyone has Sol Ring, it's all balanced. What's the harm?" The harm is that some people are going to draw Sol Ring, and get the huge benefits, and some won't. Every game will swing by a significant amount just by who's lucky enough to pull a Sol Ring in the early game.