Thoughtsteal

Thoughtsteal is a common priest spell card, inducted into the Legacy set, formerly from the Classic set.

Strategy
Thoughtsteal is unusual in that it allows you to draw cards from your opponent's deck. This can be useful strategically; the cards drawn can be used to deduce the style and content of the deck, and any card which is drawn by the priest is shown to currently be in their opponent's deck and not in their hand. On the other hand, this card gives you randomly selected cards from another player and class's deck. This can result in drawing cards that have little synergy with your own deck, or even little or no actual use, such as drawing Deadly Poison (unless you also steal a weapon card). However, playing Thoughtsteal does result in a net gain of one card, making it relatively efficient.

If a priest uses Thoughtsteal against you, you can use knowledge of the cards remaining in your own deck to deduce which cards they may have drawn.

One reason to include Thoughtsteal in a deck is simply to enjoy the unpredictable variety of drawing cards from another class mid-game. This can add an enjoyable element of RNG to the game, and defeating a player using their own stolen cards can be particularly entertaining.

If you use a golden Thoughtsteal, the cards you copy will become golden even if they are plain in your opponent's deck. This has no benefit to you when using them, other than aesthetic appeal. However, if you use a non-golden Thoughtsteal, the "goldenness" of the copied cards will follow the "goldenness" of the original cards. This may give you a tiny advantage. For example, if you copied a golden Lava Burst, then witness your opponent playing a non-golden one, they must be running 2 copies of that card in his deck, one golden and one plain. For this reason, top players do not mix golden/plain copies of any card in their decks; they either run both copies golden, or both plain. In this case, plain Thoughtsteals ofter a genuine (if slight) strategic advantage over golden ones.

Lore
While Thoughtsteal does not appear in Warcraft lore, it is similar to the Spellsteal mage spell from World of Warcraft in that it allows you to 'steal' spells from your opponent and use them as your own.

Trivia

 * During alpha, this card had the flavor text "That awkward moment when you steal his thoughts and realize he's thinking about you without pants on..."
 * During alpha, Thoughtsteal used to actually steal cards from your opponent's deck. However, this was found to cause too much "emotional negativity", especially when stealing key cards from highly structured decks, and was changed to instead generate copies of the cards. This was an early example of Hearthstone embracing the possibilities of the digital medium, something which would not have been possible as a physical card game.
 * The artwork for this card comes from the World of Warcraft Trading Card Game "Drums of War" series, for the card Lose Control.

Patch changes

 * Moved from Classic to Legacy set.
 * The copied cards now retain enchantments.
 * Unknown alpha/beta patch: Flavor text changed from That awkward moment when you steal his thoughts and realize he's thinking about you without pants on... to What do you get when you cast Thoughtsteal on an Orc? Nothing!" - Tauren joke
 * Now copies 2 random cards from the opponent's deck, instead of stealing them. Card text changed from "Draw 2 cards from your opponent's deck." to "Copy 2 cards from your opponent's deck and put them into your hand."
 * Now copies 2 random cards from the opponent's deck, instead of stealing them. Card text changed from "Draw 2 cards from your opponent's deck." to "Copy 2 cards from your opponent's deck and put them into your hand."