It's like general culture, people!
I disagree. Although I have heard good things about the book and feel I should give it a shot...
But I think that a comic that relies heavily on niche knowledge (ie. anatomy text images on Wikipedia) is doing itself a disservice.
I'm not saying that xkcd is bad, necessarily. I'm saying that I'm sick of hardcore fans telling me that I need to know more about popular science fiction/string theory/computer science/engineering/philosophy, and/or read the entire archive to get it.
That's way too much work to put into a comic that EVEN FOR THE STRIPS THAT I GET, doesn't seem to be worth the payoff.
TL;DR
It really seems to me that xkcd is the kind of strip where hardcore fans can nudge other hardcore fans in the ribs with the elbow, and laugh derisively at the common simpleton who
deigns to try to read it without first having
educated themselves.