In order of descending OP-ness:
If a card is so game-breaking that it forces all decks to use it in order to compete, then it is OP.
If a card cannot be countered by anything whatsoever, then the card is OP no matter what.
If the card only has one or two counters, then it is OP, but not as much as the one above.
If a card is strictly superior to another card, then card 1 is OP unless card 2 is UP. I think only a bit of playtesting can determine this one.
If a card is too cheap for what it does, then it is slightly OP, but not as much as the ones above.
Some effects are frowned upon by the community in general (e.g. instant killing, forced discard). The community often regard those as OP.
Anything else?