IN HOLD'EM, POSITION IS POWER
But in Texas Hold'em, position is immutable. The player who holds the dealer button KNOWS without fail that s/he will be able to act last on all rounds of betting, as long as s/he remains in the hand. That's a very powerful advantage, much more powerful than most beginners realize.
Similarly, the two players who have to post the small and big blinds know that they will be OUT of position the entire hand, that they will always have to act first, which is a very significant disadvantage, and again, probably much more significant than most beginners realize.
Because the blinds must post their money before they see any cards, it's quite possible, even likely, that they will be entering a pot with both an inferior hand and inferior position. For this reason, it's very common for the player on the button, holding the desirable final position, to raise if no one else has entered the pot, thus "attacking the blinds."
MOST PLAYERS DEFEND THEIR BLINDS TOO MUCH
Even though the players in the blind know that the button may well just be pushing a positional advantage, and may not have a real hand, they are often quite right in releasing (folding) most marginal hands, because they will have to play them out of position.
Because the first player after the button posts only the "small" blind (usually half, occasionally one-third or two thirds the size of the big blind), his decision to fold when the button raises is usually fairly easy. With only half a bet in the pot, a player has to have a pretty decent hand to put in another bet-and-a-half, knowing that he will be out of position the entire hand and also facing the possibility that the big blind may re-raise.
The big blind's decision is simultaneously tougher and easier. It's easier because there's no fear of another raise before the flop (the small blind has to worry about getting popped by the big blind), and because with a full bet already in the pot, and the small blind's folded half bet already sitting there as "dead money," the big blind knows he only has to spend one bet to chase the three and a half bets already in the pot (his original blind, the small blind, and the two bets put in by the raiser).
On the other hand, the big blind's decision can be harder, because the price of the call seems so cheap, it can be easy to get trapped into a hand where you can lose a lot more money.
THE BIG BLIND GETS GOOD POT ODDS IN DEFENDING, BUT…
Getting 3.5 to 1 sounds pretty good, and so even when the big blind is pretty sure he's trailing, it's often worth a call to see if the flop hits your hand. But because of the positional disadvantage, a lot of hands are not worth calling with (usually called "defending").
Most players learn pretty quickly that hands like 7-2 offsuit are not worth defending, and that hands like J-Q are. The less intuitively obvious hands are the smaller connectors, like 5-6, and the hands with one good card and one bad one, like K-4 or Q-5.
Although not everyone would agree with me, I'm much more willing to defend a hand like 5-6 than I am a hand like Q-5. The Q-5 can't work together to make a straight, and even if a queen does hit the flop, there's a significant chance that on any hand where there is action, the raiser has a queen with a better kicker.
"ROTTEN LITTLE" HANDS USUALLY MAKE FOR EASIER POST-FLOP DECISIONS
The 5-6, on the other hand, can flop some very tricky, hard-to-see-coming hands, and even better, it's usually very easy to throw it away if you miss the flop. Suppose you have Q-5 and the flop comes K-Q-8. Do you have the best hand? It's very hard to know, and you're probably going to be left guessing for several more bets. If you have 5-6, and the flop comes K-Q-5, although you MIGHT have the best hand, it's very easy to throw your hand away.
Even more importantly, if you have 5-6, you can be up against a monster hand like A-A or K-K and still come out beautifully. If the flop happens to come down something like 2-3-4 or 4-7-8, you are probably going to win a LOT of extra money from the pre-flop raiser. This is what we call "implied odds"—money that you stand to win in the future if you make your hand.
It's far easier to trap a big hand with two cards that work together than with two cards that don't.