When I took high-school physics, my teacher explained (rightly or wrongly) that a lightning rod does not attract lightning. A lightning rod is pointy on top, which concentrates its electric (or was it magnetic?) field-lines at one point, which makes it possible for it to discharge electrons steadily and harmlessly, from the ground into the air, thus preventing the charge buildup which leads to a stroke of lightning. A van der Graaf generator, by contrast, has a globe on top because that shape holds the charge on the surface and prevents it from dispersing, thus it builds up a charge that we see discharged in sparks when you put your finger close to it.
I have always believed this to be true, and in consequence I have always been annoyed whenever I hear or see the phrase “lightning rod” used as a metaphor for something that draws trouble to itself. I recently mentioned this in a nitpick in a GD thread – http://boards.straightdope.com/sdmb/showthread.php?t=273839 – which quickly turned into a hijack. Several Dopers joined in and insisted a lightning rod does attract lightning, therefore the metaphor is an apt one. They cited several physics sources. What’s the Straight Dope?