There are a handful of cards in Hearthstone - often Legendaries - which can often turn the tide of battle around completely. Sylvanas Windrunner is not just a solid minion to put down on the board, she also steals a random enemy minion for you when she dies. As you can imagine, this can cause your opponent all kinds of headaches, as they rush to remove the creature and minimise the risk of their own losses.
Most people - newcomers and veterans alike - over-think the use of Sylvanas and try to wring maximum efficiency out of playing her onto the board, often delaying use of her until the right moment has long passed. The truth is that you can gain just as much value by obstructing and confusing your opponent into making either inefficient or poor plays, as you can by stealing that perfect minion you've been holding out for.
Here are a few interesting ways of making use of this powerful card. Have a think about which one suits you current situation, and make sure you follow up the initial play accordingly.
Put a cheap Taunt on Sylvanas and make your opponent sweat
Nothing turns the average player into a ball of shredded nerves like a cheaply Taunted Sylvanas. Your opponent has to calculate the best way of dealing with the problem, but without overloading their board and giving you a juicy free gift. Always try to maximise the annoyance factor of this card and force the other player to make terrible plays to eliminate the threat. As the clock ticks down, their decision-making will become worse and worse.
Use Sylvanas to force your opponent into making really, really, terrible plays
Don't feel that Sylvanas has been utterly and comletely wasted if you don't get a fantastic minion from your opponent when she dies. More often than not your opponent will be forced into making either terrible trades, or playing multiple cards, in order to remove her from the board. Value takes many different forms - don't be afraid to use this card to get a decent dollop of it out of your opponent.
Kill Sylvanas yourself to get a clean shot at a target
Do you have the means to destroy Sylvanas completely? If you have the means to eliminate Sylvanas from the board (Shadowflame, Shadow Word: Death etc.), and you have an unspoiled shot at a powerful target minion, consider nuking Sylvanas yourself to grab the opponent's most awesome card nice and cleanly.
Use Sylvanas to bait out an opponent's hard removal
Another use for this card is if you have a powerful game-winning minion in your deck, but you don't want it to perish immediately to hard removal spells like Hex or Polymorph. Throw Sylvanas under the bus instead, force your opponent into an inefficient turn, and then drop your win-bomb onto the board.
Sylvanas is probably your best play, even if there are no targets
Unless your curve works significantly better by making a different play, just put her on the board if you can afford to do so. Just the sight of this card will throw your opponent's game into disarray, slow them down, or force them to do something they'd simply rather not - it's a great way of driving another wedge into the psychological war that underpins every Hearthstone match.
Increase your chances of Sylvanas hitting the jackpoit
This is perhaps an obvious point to make, but be sure to trade off other minions smartly so you increase the odds of Sylvanas getting a juicy target if you intend to kamikaze her into something as your final play of the turn.