I think every card is playable, which kinda is the charm and strength of M:TG. Only those cards which provide effects that can be gained for less costs in the same colour should be deemed unplayable I think, and even then most of the times it is there because the other card is a rare.
However, some cards are more playable than others of course... Manabond, Relic Bind, Pale Moon, 98% of the Homelands expansion... I suppose these are cards that should qualify as "unplayable" (unfortunately, I possess multiple copies of some of those
).