Most good players know when they call preflop and when they fold. This decision that is made preflop can be crucial for the further game. Of course depending on the table you are playing and also on the character of your opponents you should adjust and vary your poker game as much as possible.
However, in general you should always play strong hands preflop, because a medium strong hand will only cause you trouble. Cards like K4 are to be thrown away at any position. Maybe Heads-Up it could be played, but that is another story.
The described preflop situations are very important. Still, that is not all what matters! After that you may have to make three more important decisions. Of course theories like Pot Odds can help you to make a good decision, especially if you need one more card for a straight or a flush, but what are you going to do if you already have a strong hand and you don’t know if it’s good enough to beat the other ones?
Here is one example:
You have AQ and the flop shows KQ5. Somebody did raise preflop, so how are you going to bet on the turn? This depends a lot on the other players moves. If one player is making a bet, another is raising him and the next player calls it then you can be sure that your pair of Q won’t have a chance.
This is just a small insight in the world of important poker decisions.