Position

Position is extremely important in poker.  Generally the most favorable position to be in is late position, on the button, though occasionally being in early position and being able to bet out first can be an advantage.  Most people know that late position is great because you can see weakness ahead of you and make steals.  What people often underestimate is how valuable late position is when you have a hand as well.  Overall you can lose less when you lose and win more when you win, because you have more power to manipulate the pot.

Consider a marginal hand.  You play QJ and the flop comes AQ5.  You have middle pair, but you're afraid of the ace.  In early position on the flop, you're in a bad spot.  If you bet, you will get called or raised by an ace, and any hand you beat will probably fold.  If you don't bet, you set yourself up for a bluff because you've shown weakness - now the dealer will bet even if he doesn't have an Ace, and you have to fold.  Trouble all around, certainly if you do win the pot, it will be small because you can't confidently bet.  On the other hand, playing from the button, you have many nice choices.  If someone comes out betting you probably just fold unless you suspect a bluff.  If it checks around, you can be pretty sure there's no ace (obviously this all depends on how much strength was shown pre-flop).  You could check and try to induce a bluff on the turn, or you could bet for value, because many people will assume you are stealing and call with marginal hands.  It's much harder to trap and raise you.

The difference in what you can play with position is really severe.  There are two big factors here : 1) seeing the preflop action before you decide what hole to play, and 2) just that playing in late position is so valuable you want to play more hands.  Factor 2 is especially big in No Limit Hold'em, where position matters even more because it is such a psychological game.  Playing in late position, you can easily win the pot with any two cards just by seeing weakness in the play ahead of you.  Certainly once it gets short handed, being on the button is so valuable that you want to play nearly any two cards (assuming it hasn't already been opened for a raise).

The preflop factor is that in late position you have seen how many people are in the hand and what kind of strength they have shown.  You get to know how many people are in, and what kind of strength they have shown.  Hold'Em is a game either of drawing or dominance.  If you are considering drawing, it's a huge advantage to know how many people are in.  For example, a hole like 22 is a drawing hand, trying to make a set of 2's.  With several people in ahead of you, it's a good call to see a flop on the draw.  With only one or two people in, you're getting bad odds on your money, and you should fold.  In early position, you can't know what will happen, so it's best to just fold.  Similarly, hands like AT or AJ can be very good if there are no higher aces out, but if someone has a higher ace then you are badly dominated.  Seeing the betting action before you decide to play is enormously helpful in guessingwhether you are dominated.  These hands should almost never be played from early position, but they can be played from late position if everyone folds to you or you have reason to believe they're on weak hands.