I've learned so much over the past few months, and especially in the short time I've been a member here, I don't believe it's possible to narrow it down to just one thing. For me personally, in addition to the 3 below, I'll add patience, accepting bad beats and not let them cause tilting, the absolute importance of position, and 3 & 4 betting in cash games.
Originally Posted by wanderingthehall
SNIP<<<<1. stealing blinds 2. opening pots 3. properly shoving when short stacked.
Now, if I had a choice of 1 more thing to learn (and probably master), it would be how to deal with slow play. It's the one bad thing now that I know seriously affects my game.