Assuming a grasp of poker theory, the fundamental strategy of the specific game being played, and a basic understanding of poker math, I would say the most important skill in poker is the ability to understand the psychology of your opponent.
This psychological understanding incorporates a range of skills, such as bet-size pattern recognition and adapting a strategy to counteract another player’s strategies. It means knowing what your opponent is likely feeling and thinking and how he is switching up his game in response to what has just happened. This is something you can only get through years and years of playing many hands against many types of players.
For those wondering such questions, this is where someone like Phil Ivey differs from other good, but not great, winning players. He has an understanding of his opponents and their likely strategies, and can adapt his play to take advantage of how they’re continually adapting to his play.
This is the reason why Ivey can dominate both online and live games, tournaments and cash games. For someone skilled in understanding their opponents’ actions, the specific poker game doesn’t much matter. A new game can be picked up relatively easy for someone generally knowledgeable at poker theory. People can learn the mathematics of a game in a few months (assuming above-average math skills). People can learn the fundamental “correct” strategies of a game just as easily.
What people, including some very good players, cannot easily do is understand the complex interaction between players: how your image affects your opponent’s play and how your opponent’s image affects your play and how these things continually interact back and forth. There are few people who reach a greatness at this level, if only because I think many of them do not know that greatness is truly possible at this skill. Also holding people back is the fact that you can become a significant winner without ever really reaching mastery of this skill.
This is why the endless analysis of single hand histories on a lot of websites can be so tedious. If you have an advanced level of skill at the fundamental strategies of poker, then a single hand is all but meaningless. What matters is not how that single hand is played (unless there is obviously a fundamental strategy flaw), but how that single hand fits into the ebb and flow of the game. Almost any play of any hand could be theoretically justified if you created the right game conditions around it that would justify that play.
What matters when talking about a single hand is a whole slew of hard-to-describe interactions leading up to that moment. The ebb and flow of the game. The switching of gears. The ups and downs in money that influence how opponents act. This might be the main reason you won’t find Phil Ivey talking strategy or posting hand analysis on TwoPlusTwo: he might honestly not understand how people can be talking about single hands in isolation.
So this is what I mean when I talk about the psychology of the game. It is a subject I’d like to examine on this website, and it is not something I pretend to have much proficiency at. Assuming a great understanding of a specific poker game (which is the easiest part of poker in my opinion) it is this psychological understanding that bears the most examination, and for many reasons it is also the hardest to examine.
We simply don’t have a language for the thought processes that go on in the minds of great players like Phil Ivey. They don’t have a language for it either because it is something earned from so many years of play and conscious (and unconscious) analysis; but it is this skill that separates the great from the very good.