Hearthpwn

I've done a fair bit of testing with different archetypes and am pretty comfortable with this list. The 2 drops are INCREDIBLE for early game.

The coin should ALWAYS be saved. Against control you can coin out your 3 drop dragon. Against agro you use it as a free minion AoE removal on turn 2 against the prevalent paladin 1 health. 

Mulligan for wild pyromancer, Explosive sheep, Mad scientist, Arcane Explosion against paladin. If agro, GTG. If control, no harm - you have time to burn. Against warrior classes ALWAYS go for the 4 drop dragon mulligan and mad scientist for an early duplicated Twilight Guardian.

Save polymorph for Sylvanas in ALL matchups. Rend is AMAZING vs. most decks. All decks, even zoo, run Loatheb/Thaurrisan/Dr. Boom.

Wild Pyro + Polymorph = Buh Bye Dr. 7

Coldarra Drake is INCREDIBLE late game, and an EXCELLENT duplicate. With him on the board, piloted shredder dies to pings. Possibly what pops out of him does as well.

Against Freeze Mage, play your Kezan mystic wisely. Pop his armor first, before stealing the Ice Block. Against Tempo mage, wait for his mirror entity to drop your explosive sheep. Same with mech mage. Enjoy the mirror sheep, AKA, free flamestrike.

Against Shaman/Control Mage, NEVER play duplicate on your dragons until they use at least one polymorph. It's the reason I don't run effigy, way too dangerous to get poly/hex and lose both duplicate AND effigy in one fell swoop.

I very much like Thaurissan in this deck. It forces decks to duplicate/deal with him. Sylvanas not as good. I ran Saraad for a while, and he is a worthwhile tech option for Kezan Mystic if you aren't facing mages/hunters/secret-a-dins/or jerks who like to run their own kezan mystic.

Polymorph is a great counter, btw, to stolen duplicates. But, Kezan is the better counter to Kezan. To save polymorph for a better time.