Hearthpwn

Hand that destroys the meta.  Board clear on turn 2 with Embrace + Circle, destroying those pesky aggro mages and winning on board control.  In general, mulligan for Embrace + Circle x 2, with SW: D as your fourth card.

Mage:

Ideally wait until turn four after they've put down both Wyrms and an Apprentice.  If they have coin, you may need to AoE on turn 3 if they get the perfect starting hand, but generally you will take around 10 points of damage in exchange for clearing their board.  You should save all your minions (hopefully you got one or two low cost ones from portal) until after blowing up the board.  Need to make a judgement call whether they have Flamewaker which means you really should wait until turn 4.  Turn 4 has added advantage that you can hero power for an extra 2 damage if they get out a 5 or 6 health minion

Rogue:

Ideal starting hand is Embrace, Circle and SW:D.  The Death is in case they get off a big Van Cleef.  Let them empty their cheap spells and then kill of Van Cleef.  This match-up is auto-win

Druid:

Ideal starting hand is Embrace, Circle and SW:D.  It is generally worth AoE'ing the alarm-o-bot if they try.  Otherwise, they'll blow all their cards to get a big minion out which you should be able to SW:D down.  If they are token and flood the board, even easier.

Paladin:

Watch out for Divine Favor if they are an aggro setup.  In this case, blow your AoE early and dump your hand.  Otherwise, they are control-ly and you win any control vs control matchup as your removal and AoE are better

Warrior:

Bait out the executes and out-control them.  They have no AoE, your control is better than theirs.

Why Sylvannas?  First, to win any jousts that might happen.  Second, it's a better entomb in this matchup.  You can put in another SW:D instead, but generally, the pressure and threat of Sylvannas is better.  Mana is not generally an issue by turn 8-10, and almost every game goes to there.