Andrew "Foucault" Brokos, professional poker player, coach, and book reviewer, writes a very interesting article titled "Stop Worrying About Draws" in Card Player magazine (October 17, 2012), the gist of which is that just because there is a potential flush or straight draw on the flop, the likelihood of someone actually having such a draw is not as high as it would appear.
He shows how for different flops the number of possible combinations of potential draws are relatively small compared to the total number of actual possible combinations.
In doing so, however, he slips up a couple of times with his calculations.
"There are six times as many ways to be dealt 9-8 offsuit as 9-8 suited, though admittedly many players will fold the former to a raise."
This is clearly wrong. There are 16 combinations of any 2 different ranks, 4 of which are suited. So there are 3 times as many ways to be dealt 9-8 offsuit as 9-8 suited.
A little further on, Brokos states :
"Two-card straight draws are almost always less likely than two-card flush draws because they require two cards of a particular rank. Whereas any two of eleven remaining hearts (110 combinations) produce a nine-out flush draw on a Th 7h 2c flop, a player must hold one of four eights and one of four nines (16 combinations) to have an eight-out straight draw."
Once again he errs. There are 55 combinations, not 110. Nevertheless, his conclusion is still valid :
"Thus protecting your hand from straight draws is generally less important than protecting from flush draws."
All in all, a thought-provoking article.