Nice list from BlueNowhere. I can add a few more...
-not playing strong draws aggressively enough
-calling 3-bets out-of-position
-not backing off your hand when a loose passive player re-raises you
-playing too many tables at once
-no table selection - you want to be at tables fish on right, nits on left if possible
-not having a stop-loss for a session]
-not having the patience to wait for good hands
-being unable to fold AA/KK postflop
-not understanding implied odds
and how to play hands like suited connectors
-ignoring bankroll management
-not reading board textures and adjusting play accordingly so you are bluffing at the right times and c-betting on the right flops etc.