Active Pieces
I played a game recently where I was coming up on checkmate and I started looking at Black's pieces to see if my plan was sound. Here, I had another moment where I gained some Novice Knowledge.
An inactive piece is just as good as a piece not even on the board. 
I saw Black's Kingside Rook sitting there and wondered if it would be able to stop my plan. I realized that the Rook hadn't moved for the entire game. Not only that, but it had spent most of the game boxed in by a Knight, Bishop and Pawn. It was useless.
In addition, by the time I realized this, it was too late for Black to get any use out of it. I had just played 21. QxB7+ and Black played Kd8. So unless Black found a way to capture my Queen or get some of my Pawns out of the way, Black's King would spend the rest of the game on the back rank blocking in its own Rook.
If Black had that Rook out, he may have lasted a few more turns or even turned the tide on me, but that piece may as well have not even been there.
So now when I look at a board and think of who has a material advantage, I try to think about not only what pieces are still on the board, but also, whether or not those pieces are in any position to do anything useful.
I guess this idea not only applies to the board as a whole, but also to small portions of the board.
For example, I played 18.Bg4 with the idea of 19.Bxd7+, then 20.Qxb6 and forking Black's Bishop and Rook.
To be honest, in hindsight, the plan probably would not have worked with Black's Queen sitting on e7, or at least not as clean as it did, but Black played 18. ...Qg5 and completely removed the Queen from the equation.
I think g5 was still a decent spot for the Queen, at least to my amatuer eyes. If given one more turn, Black could have swung in to put on some serious pressure, checks, captured material, but there just wasn't time for that now. Now, 19.Bxd7+ (almost) certainly leads to a destroyed Queenside and, ultimately, Checkmate.
An inactive piece is just as good as a piece not even on the board.
