The coolest thing in SC2 is Nightmare's bag of tricks immediately following a standing B, like when you hit down+A before the move is over and he yells 'DIE' and whacks their legs out from under them. Most people are used to Nightmare just belting out B, B as fast as they can.
After you do that against a human like 5 times, they learn to block low, so you hit another standing B instead and hit them in the face.
After they get a clue, go back to hitting down+A after B; Nightmare still yells the "DIE!!" which provides them with the aural que to block low. EXCEPT! Immediately hit BACK and Nightmare immediately goes into night-behind-stance, and then you hit B to overhead-smack their crouching ass. It's a far more effective tactic than buying SC2 a week ago and saying "I'VE BEEN MASHING WITH MAXI AND KILIK FOR A WEEK IM PRETTY GOOD I BET I CAN BEAT U" to people on IRC who have been playing for over a year.
Oh.