Huh. My referrer logs show that a couple people hit my previous post by searching for heads-up starting hand requirements. That's probably not the best way to think about heads up play. Pretty much any two cards are "playable" in heads up no-limit hold'em. A majority of the time no one will hit the board, so competing for pots becomes a game of chicken.
On the flip side, when you do hit the board it's sometimes better to be holding weak cards. When weak cards hit, the board tends to look pretty weak as well, so you have the potential to trap your opponent if they happen to choose that moment to stand up to you.
Still, not every hand has the same pre-flop value, of course. When I advocate raising with 80% of your hands, it should usually be done with your best hands. Just limp with hands that don't have high cards, can't make straits, and are un-suited. Mixing it up, of course, by occasionally limping with strong hands and raising with trash.
But position is vastly more important than your cards. Being able to act last after every betting round is a huge advantage heads up. You'll be able to steal more with your weak hands and win larger pots with your big hands.
Some players, Gus Hansen for one, prefer to be out of position heads up. Gives them the opportunity to be the first person to bluff at a pot. This may work for some, but a majority of the players I've encountered who play this way can't get away from the hand if they get raised. You have to be very adept at knowing when to back off and when to take a stand. If you're not already dominating your heads up games, it's probably better to rely on position when making moves.