5. Playing too passively (e.g choosing cramped openings without understanding the importance of "freeing moves" (i.e., pawn breaks)Hand in hand with the above, if you're choosing cramped openings where you concede a space advantage in the center or on some flank for future play against it, then you'd better understand the thematic counter play mechanisms (i.e., pawn breaks, piece placements, common tactics, etc.), else your opponent will implement his plans unopposed.
6. Wasting time (tempos) in all phases of the gameGMs are highly cognizant of wasting time by moving the same piece more than once in the opening or by grabbing minor material at the expense of development. Moves that apparently threaten something but only elicit a response that forces a retreat and a subsequent improvement in the opponent's position are avoided (i.e. pinning a knight with Bg4, Bg5, Bb5, Bb4, provoking h3, h6, a6, or a3 and then not taking the knight.). Also, watch when GMs make moves like castling, moving pawns around their kings, or re-positioning their kings (in the opening or middle game, not endgame): they almost always play these moves when their opponents last move threatens nothing. When your opponent plays a move that threatens nothing, that is an opportunity to either strike out or to take time to repair, improve, or fortify your position, too.
7. Neglecting your king's safety (castling, making-luft, removal from open lines, etc.)We've all been there: we're about to administer the coup de gras, and just when we play what we think is the decisive move, our opponent, instead of resigning, plays <insert move> check! Ugh!!! Our brilliant combination falls apart because of a devious double attack on our king and some other exposed piece. Thus, before conducting active operations, you will see that all GMs will make sure that their kings are safely tucked away. Being safe, at a minimum, means that all avenues of attack have been blocked and that the king cannot be checked. If you observe GM games, you will see them erect a kind of fortress with pawns on f2, g3, and h4 and the king on h2 (and so on symmetrically around the board). In this formation, the king is guarded from all three directions (the a8-h1 diagonal, the 1st & 2nd ranks, and the h-file), back rank mating threats are minimized, and the king has bolt-holes to escape if need be. Knowing when to play moves like Kh1 goes hand in hand with my comments above, because doing so has to balanced by the consideration that you are moving your king one square further from the center, which could be important in some endgames.
To these items, I could add:
From what I've read and observed, attention to these kinds of details is the shortest path to rapid and significant improvement. One thing that I keep reminding myself (because I still have this dream of reaching 2200+ myself) is that chess mastery is all about the degree to which you've internalized the basics such that you follow them without much, if any, conscious thought -- much like a golf swing or batting in baseball. You just do it until the mechanics become instinct. That only comes from lots and lots of play and analysis with stronger players.
I'm always interested in playing better myself, so I'm happy to chat about these points with folks at the club anytime! - Jason