There's a lot of things I had to learn in order to really step up my game. Off the top of my head and in no particular order...

1) The value of position and its relation to your starting hands.

2) Pot Odds - HUGE difference maker between an okay player and a great one.

3) When to bluff. I used to be way too tight and only play super premium hands. These are nice, and tight is still the way to play... but sometimes you just have to go for it and hope your opponent backs down.

4) Not to get tilted and try to make back big losses immediately. It's a slippery slope.