There was a question asked a bunch of players at the WSOP in Bluff Magazine recently. What mistake is most often made by bad players?

The predictable answer was always "plays too many hands".

That might be the most frequent mistake, but I don't think it's the biggest mistake. By itself playing too many hands doesn't really cost you much money, it's what happens after the decision to play that costs you the money.

There are two really expensive mistakes that I think are much worse than playing too many hands --- calling to many raises, and too much of a focus on trying to win the pot.

For many hands a raise by another player indicates a high probability of being dominated. That's a really, really bad situation to get yourself in. Don't call those raises unless you have either a very strong hand (after considering the hand he probably has to raise with) or are sure you aren't dominated and the remaining money is still deep.

The excessive focus on trying to win the pot tends to cause players to get much, much deeper into a pot than their hand or the situation warrents. Especially early in the betting rounds, when the pot is still small, a focus on gathering information, observation, and carefullness is much more important than trying to win the pot. Even later in the hand, when the pot is big, too much focus on trying to win just often gets you in trouble.
Best at the table.