This is another question I was NOT asked at my poker talk last month, but that I figured would make for an interesting blog post.
This question is basically the same as the following: what do you do as a poker player that sets you apart from most other players and enables you to win? It's not strictly necessary to be able to answer this question in order to be a +EV player. For example, it could be that your income comes entirely from the 5% of players who are truly terrible, and that against most players you have no real strategic advantage. In practice, though, a winning poker player will try to identify and exploit weaknesses in all his opponents.
Below I list several common weaknesses of players. You will notice a common theme of players being conflicted between the "correct" goal of maximizing EV and some other goal, usually related to psychological satisfaction. I believe one of my primary advantages over other players is that I am not an emotional player. To the extent that my emotions are involved in my play, they are usually tied to my ability to make +EV plays, since it gives me satisfaction to think I am playing well.
The most common weakness of players is to play too many hands. Any player who has heard even a little poker advice knows that they should be playing very few hands. To a new player in a nine-player NLHE game, it is shocking how often they should be folding, and it often takes a while for them to really believe get used to it. Among weaker players, I think another problem is that they have trouble reconciling conflicting motives: they want to make money but they also want to have fun. Folding is not fun, so they play lots of hands even if they know they shouldn't.
Another common weakness I see is that players overestimate their skill level. This often manifests itself in the first weakness I mentioned: playing too many hands. Players think they can make more hands win than they really can. In good players, overconfidence can also result in "Fancy Play Syndrome," which means deviating too much from standard play, usually in an attempt to make a "hero call" or a big bluff. These plays are psychologically extremely rewarding when they work, which suggests that good players, too, succumb to conflicting motives.
A more specific weakness is that players are reluctant to call with hands that probably losing but are +EV because of good pot odds. A classic example is if you hold QQ and you figure your opponents has raised with AA, KK, or AK. You are behind your opponent's range here but you will likely want to call if you are getting good pot odds. (This is one reason why I am a big fan of semi-bluffing with AK before the flop.) Again, I think this can be traced back to a problem of conflicting motives. Good players often care deeply about their reputation, and since it can be embarrassing to call with a hand that is behind, they are reluctant to do so.
Finally, I think good players spend too little effort considering their own hand ranges and too much trying to "outthink" their opponents by thinking one "level" higher. It's much more efficient to just make sure you have balanced your play in a game-theoretical sense. For example, before you check on the river with a weak hand consider your hand range given the play so far. If your hand range contains some hands that you would like to value bet with, it should also contain some hands that you will bluff with. In fact, for those who saw my talk at Swarthmore or who have read The Mathematics of Poker by Chen and Ankenman, you know that there is a specific ratio of the number of hands you should be betting to the number you should be bluffing. Unless you are trying to exploit a suspected weakness of your opponent (for example, you think he calls too much on the river), you should balance your range in agreement with an optimal strategy. (As an aside, I think that this approach is one reason why people's assessment of my play tends to be polarized, with many people thinking my strategy is carefully measured, and other thinking I'm maniacally aggressive because I find opportunities to bluff in situations that most players wouldn't bother. These assessments seem contradictory, but both are essentially correct.) I think this weakness usually stems from the fact that most players simply haven't learned to think about poker in this way. However, for some players it may be that this is yet another example of conflicting motives like we saw in the first few examples. That is, it is certainly fun to try to outsmart your opponents (rather than just balancing your own range), and so it seems likely that players will put a bit more effort into that aspect of the game than is really called for.