Slow playing AA or KK preflop can get you into a lot of trouble. Especially in early position, if you limp with those kinds of monsters and about 3 or 4 others limp in behind you, you're probably already in trouble after the flop because someone else probably got lucky with 2 pair, or they may have flopped a str8 or a great str8 or flush draw and now you've priced them in to hit that draw.