To elaborate on the subject, playing myltiplayer, you want to look out for two basic concepts of card game:
-Escalation;
-Card advantage.
Escalation are cards that get better with more players out, like the aforementioned
Sleep, Caradhras and
Saruman's Power. Those are USUALLY pretty easy to spot, but keep an eye for sleepers, cards that are TERRIBLE on one-on-one gameplay, but escalate to craziness when paired against multiple opponents.
Card advantage is a little bit trickier to grasp, as it means a HUGE amount of things at the same time. Escalators are good examples of card-advantage-producing cards, but other strategies and cards provide that.
Goblin Swarms is card advantage, since you can replay the minions stacked there and preserve the ones in hand, or play the ones in hand and get to cycle, drawing more cards. Besiegers as a strategy produce card advantage in a similar way, stacking minions and getting "more" out of them. I could elaborate longer, but I'm on a short clock right now.