After this post, I'll be more than half-way through my analysis of the sixty concepts at the end of No Limit Hold 'em: Theory and Practice by David Sklansky and Ed Miller.
Concept No. 30: Implied odds are a critically important decision-making tool, but always be aware that different opponents offer different odds.
I like this one. Players often overestimate their implied odds by assuming they will win their opponent's entire stack if they make their hand. In reality, of course, sometimes the other player will fold. Other times, you will make your hand only to see it lose to a better one. The probability of either of these two things happening can be approximated without any knowledge of your opponent. However, as Sklansky and Miller point out, if you do have any prior knowledge about your opponent, you'd better take it into account. It can make the difference when deciding whether to call with a drawing hand.
I also like the authors' observation (in their discussion) that the players who offer the least implied odds (because they will fold if you hit your draw) will also be the easiest to bluff. This slightly increases the EV of calling a player who offers low implied odds, but it also influences how you should play your draws and the types of draws you should be calling with. First of all, against such players you should be much more inclined to raise as a semi-bluff. Not only are they more likely to fold, but you're also giving up less in the way of implied odds had you just called. Second of all, you'll want to look for opportunities disguise one draw as another. For example, if you're heads-up against such a player on a board of Ad Th 8d, it's probably better to have the 97 straight draw than the diamond flush draw. You have fewer "outs," but with the straight draw, you not only have decent implied odds for the times you hit your straight, but you'll also probably win the pot by bluffing if another diamond comes. On the other hand, if you have the flush draw, your only chance to win is probably to make your flush, and if you do, your draw is so obvious that you have little chance of getting paid off. This leads directly to the next concept, so let's move right along...
Concept No. 31: Your implied odds with any draw will be better the less obvious the draw is.
Since you just read my analysis of Concept 30, you can guess that I agree with this one. If your opponents are worried about a particular draw and it comes in, they are not likely to pay you off if you hit it. Against some players, you'll probably want to make only a tiny bet if you hit such a draw, because that is your only hope to get paid. In other words, if your draw is obvious, you have very little implied odds. S+M use the example of the nut flush draw on a flop of three diamonds. Everyone is worried about the flush. On the other hand, if you decided to play 53 and the flop comes K42 rainbow, you will have very good implied odds for drawing to your straight.