very interesting topic :-) I will try to add some thoughts, hoping it will remain brief enough to be of some use.

I feel there is a lot of truth to what ih8sens writes for the range up to 1900 or 2000 rating (OTB). And to Gonnosuke's observation that up to about that strength it is a lot about tactics.

On all levels, some things can vary individually, of course. But maybe an interesting try to describe the steps that can follow could be:

(all that follows refers to OTB FIDE ratings)

2000-2200: when playing games with players of that strength and you calculate some difficult tactics, you will notice very frequently that your opponents has been able to calculate all of these critical lines, too. So "being good at tactics in general" does no longer make the difference. Then what starts to make the difference at that level?

You start to use the opening and the endgame for fighting. Before, you might have played the opening moves you read in books, and you might have played many endgames thinking of the elementary endgames that you might have known from the books to be winning or drawn.

But now, you notice that you start to wish to choose your openings so that they match your personal strengths more than those of your opponent, and you start preparing the choice of your openings against your oppenents before the games.

And you may notice that the players you meet at that level are fighting in the endgames just as they do in the middlegames. They do not just try to follow what they might have read in endgame books, but they fight and think and try to create problems and difficulties for their opponents, and they try to find plans and ideas and new tactical motives even in these seemingly "endgameish" positions.

2200 - 2400 or so:

I have experienced that players who reach that level start a new level of fighting in the middle game. They train to obtain a deep understanding of the plans and ideas in the typical middlegame positions that they reach with their openings and with their way of approaching chess. They start to have many types of positions that they are very good at, that they have understood very deeply, in which they may start to feel quite easily where the pieces belong, which ideas may be most effective, which nuances may be decisive, which candidate moves come to their mind most easily. They can calculate ahead more easily because they need to consider much fewer candidate moves because of this deep understanding of "their" types of positions.

2400-2600 : I think players at that level not only have an even deeper understanding of "their" types of positions, but their level could also be characterized by a considerable broadening of this deep understanding. If you only played one opening line and exactly one type of positions, your opponents would often have an easy time to find positions you do not like or to outprepare you by finding and learning concrete ways of how to play against your particular setup.

And players of that level have to find opening novelties, improvements over what has been played in all games so far, in order to confront their opponents with new problems.

2600 and above : In addition to the things mentioned for all the levels below (among them finding ways to work and play that match your personal strengths and abilities, and that turn out to be effective weapons against other players of that level), one aspect of expertise of these players is energy and quick learning. Energy to fight on the board and to do all the preparatory work that is necessary to keep your opening weapons sharp and your arsenal of opening weapons replenished. And quick learning in order to be able to absorb much of all the new concepts and ideas that are developed by all the other top players in their race to be a step ahead in the knowledge of opening weapons and in the deep understanding of positions that arise from them. A novelty played in a tournament game on one day may have lost most of its value of being fresh and surprising on the very next day in another tournament, because the opponent may already have seen and analyzed it in the meantime.

@artfizz (Responding to post #50) The CM title, while in spirit closer to the Expert title, in strength is closer to the NM title. From what I know: USCF 2200+ is NM. FIDE 2200+ is CM. My perception is that it is more difficult to get 2200+ FIDE than 2200+ USCF, which would make the CM title more difficult than the NM. I suppose the problem is that "master" in FIDE indicates FM (FIDE 2300+), but "master" in USCF indicates NM (USCF 2200+).