High Unpaired Hands
There is a tremendous difference between the various high card unpaired hands that you will be dealt before the flop in No Limit Hold'em. AK is one of the strongest hands possible, and you can reraise with it profitably against almost any player and re-re-raise (4 bet) preflop against many opponents but not all (and in some games, not even against most). AJ on the other hand should be folded to a raise against almost any opponent. K10 should be folded no matter what the position or action.
For the most part unpaired hands are not playable. The exception here is when you are in late position and everyone has folded. In this spot virtually any two broadway cards can be playable. By contrast if you are under the gun, I recommend limping AQ and folding it to a raise, if not just folding it initially. On the button you can make a case for open raising K7o but even KQ should be routinely folded when you are the first player to act before the flop. Against a raise the weakest unpaired hand I generally consider playing is AQ, although against some maniacs weaker hands can be profitable.
Some players like to call with A10 or AJ, especially after limpers and while in some circumstances this is profitable, you have to ask yourself - if your hands are worth playing, aren't they worth raising? Either you have a stronger hand overall than your opponents, so you should raise, or you have a weaker hand, so you should fold. This is not always the case with so called drawing hands, like a small pocket pair or a suited connector, but it's certainly the case for hands like AJ or A10 that will not frequently outdraw a stronger hand. The weak play of these players (limping in instead of raising), is indicative that they recognize the worthlessness of their hand but lack the discipline to fold it.