You have no right to cancer other decks if you play Midrange Shaman, even brutal turn 4 win face aggro pirate. And yes, until Renolock/Goon Paladin and Dragon Priest become meta (if), it should be still easy to win with it.
Nonono) "Cancer" is a deck which goes strictly to the face no matter what, like pirates or say aggro hunter at the times of HS beta) Midrange shaman is a deck where you have to think. Time to time :D
Actually, "cancer" decks are those that have spread throughout the meta to such an extent that it chokes off a healthy variety of parity between classes and archetypes.
Because they are usually cheap and relatively easy to play, face decks often become cancer decks, but they are not necessarily the same thing.
Midrange Shaman in the current meta and Secret Paladin in the pre-standard meta are examples of other cancer decks that are not face decks.