Personally, I think you're all looking at it wrong.
The number of cards that the card has a synergy with isn't the question. The question is the number of decks that are either created or altered by the card's existence.
Blessing and SoW may combo with >every creature<, but it only makes a certain number of viable decks. (Last time I checked SoW works well in at least seven decks.)
See, the most parasitic of all cards is simply one that only works in one deck. That's an issue. Why have this card that alters only ONE deck, when you can have this card which not only alters many, but creatures some of its own?
This is also why Cat is, theoretically, not an issue. It may only combo with... a tiny handful of cards, sure. But it makes more than one deck from that tiny handful.
At least, it's supposed to if it wasn't so weak.
This explanation also explains why cards like purify and reflective shield - essentially just semi-hard counters to certain decks - are fine, even though they don't really... Combo.