When she dies, take control of an enemy minion. Not many cards in the game will allow you to do that. Mind Control is the obvious one with Mind Control Tech being another. The funny thing about this card is that it's pretty amazing to watch players squirm and make weird plays to ensure that Sylvanas doesn't take over a random minion of theirs. I've seen players intentionally kill their own big guns by throwing them against my taunt minions. Sometimes they'll even kill 3 of their own minions and wipe out their side of the board just to explicitly make sure I don't gain control.
Deathrattle: Take control of a random enemy minion.
Think about that for a moment. Players are willing to trade 3 of their own minions for 1 to ensure that I don't steal their stuff. She really can mess with your mind.
Playing with Sylvanas Windrunner
At her core, Sylvanas is a card that punished your opponent for killing her. Your opponent can kill the 5/5 but only if they're willing to trade it for a random minion they have at their disposal. I would happily let Sylvanas die in exchange for a shaman's Al'Akir or a hunter's Savannah Highmane. Opponent has a juicy legendary in play? Try to use your spells or minions to pick apart the rest of theirs. As a mage, you can use the mage hero power to sort of "ping". I generally don't do this unless the opponent's legendary is something really awesome. In most cases, Sylvanas is an excellent threat just by herself.
Playing against Sylvanas Windrunner
Silence effects, guys! Anything you can use to silence Sylvanas to prevent her from stealing your Ragnaros or your King Krush! Do it! Alternatively, take a stock of your hand and the board. Is there anything worthwhile for Sylvanas to steal? If all you have is a Searing Totem (from Totemic Call) out, killing Sylvanas in favor of your opponent taking a Searing Totem isn't a bad idea. On the other hand, if you have a valuable minion on the board, you might not have a choice. Paladins and shamans can generate random minions with their hero power to hopefully get Sylvanas to take control of one of those.
Suppose the worst case scenario happens. You lose your bomb creature to Sylvanas. Analyze your hand or your deck and see what other removal spells you might have at your disposal. Obviously if you can Polymorph it or Hex it, you should. But if all you have is damage spells, you might have to save it and execute your own minion to prevent your opponent from taking command and benefiting from it.
Check out other previously featured cards!