The most important factor when deciding whether a starting hand is playable is position. Quite frankly, certain hands that are very playable on the button are an easy fold under the gun.

Generally speaking (though certain playing styles will dictate differently, see some of my previous answers in this category), very small pairs, ie., 22, 33, 44, should be folded in a nine-handed game when called upon as the first to act. Ditto small suited connectors and large unsuited aces.

Your hand selection can loosen considerably based on how many people have folded before it is your turn to act. For a very good look at the "starting hand requirements," look to "Small Stakes Hold 'Em" by David Sklansky.

Another thing to take into consideration is the so-called "gap concept," which dictates that you need a stronger hand to call a raise than to open-raise yourself. You might like your AT in the cut-off, but if someone in early position has open-raised you may want to fold it as you are likely looking at a pair or AK/AQ, any of which put your AT at a distinct disadvantage (especially the latter two). If there has been a raise and a reraise before you act, you better be looking at a premium hand to even consider seeing a flop.

You must also consider how many players have entered the pot before you. In a family pot with five or six players looking to see a flop, that same AT does not look nearly as good as 67s. Certain hands have an advantage against a single opponent (large pairs, AK), whereas others are very good in large, multi-way pots. This is because these kinds of pots often grow large enough to give the correct odds to chase your draw to a straight or flush.

This is only the beginning, though. You must also take into consideration what you have observed your fellow players doing on previous hands. If an absolute maniac who has open-raised a few times previously and then shown down weak holdings (K3,J9, etc.) open raises again, you could be more likely to call him with your AT offsuit.

Many of these nuances are addressed in Small Stakes Hold 'Em, or any of the 2+2 books. If you're playing No Limit, try Super-System by Doyle Brunson, but don't stop there. Read everything you can about the game, gather the lessons of the pros (and on internet forums from the not-so-pros) and develop your own unique style of play which your opponents will find hard to pin down.

Good luck.