The popular appeal of Randall Munroe’s totally awesome webcomic is limited by the fact that so many of the jokes depend on obscure knowledge of science, math, computer science, computer gaming, etc. But the Explain XKCD website gives the lie to the general principle that no joke is funny if it has to be explained! 
There is a forum for discussing xkcd strips. Unfortunately, it’s been off the air for several days now. Lets hope it returns soon.
So I just want to say that today’s (9/13) strip is spot on. I get so tired of announcements of “habitable” exoplanets, where the planet may be (but most likely isn’t) more habitable than Mars.
“Obscure knowledge”? I’m not even a graduate of a real university due to military service and having taken a shortcut with the piece of shit University of Phoenix, and there’s nothing “obscure” about the knowledge needed. Are people really stupider than I am?
Some are pretty obscure. Consider this one. You have to know that there are ancient writing systems called Linear A and Linear B. And that there is a technique called A/B testing used to test which of two variants (say, of a web page) is more effective. I think the majority of the public would not know of even one of those, let alone both.
Though Munroe got it wrong–the implies that the user saw both variants, and is expressing a preference. But real A/B testing only presents a single variant to any given user, and then uses non-subjective metrics to pick the more effective one.
I’ve seen a number of strips that require uncommon knowledge. Examples from just picking a few at random:
xkcd: Location Sharing requires knowing a bit about the Heisenberg Uncertainty Principle.
xkcd: Win by Induction I actually don’t understand – I guess it requires knowing something about a video game or cartoon or something.
xkcd: Voice Commands requires knowing what the Dvorak keyboard is.
xkcd: Fundamental Forces requires more than a passing knowledge of the four fundamental forces (at least to really appreciate it).
xkcd: Query requires knowing some SQL.
xkcd: Exploits of a Mom requires knowing even more SQL.
xkcd: Ineffective Sorts which I find absolutely hysterical requires more than an elementary knowledge of coding.
Huh, at least two of those, I hadn’t seen before. I hadn’t realized I’d missed any days.
Oh, and to explain the video-game aspect: The funny-looking creature is Pikachu, the best-known of the Pokemon. In Pokemon (any version, card, computer, or cartoon), players don’t battle each other directly: Instead they summon creatures (the Pokemon) with various abilities, and have the Pokemon battle each other. This summoning involves releasing the creature from a small spherical container (a Pokeball), and saying something like “<creature name>, I choose you”.
Monroe is here positing a version of Pikachu whose special ability, instead of the usual lightning zaps, is to itself summon a Pikachu. So the original player, instead of having one Pokemon to fight his battles for him, gets an arbitrarily-large number of them, with which to overwhelm whatever his opponent might summon.
I got that one just fine without knowing anything about SQL. The joke is that the parent gave their kid a name that fucks up databases. You don’t really need to know why “drop tables” would fuck up the database.
And the alt-text requires some knowledge of exponential growth. Not quite obscure, but a significant minority of people understand it at an intuitive level.
If a Pikachu cycle takes a minute, then after ten days you’ll have about 15,000 Pikachu.
If every 40 of them are doubles, then after ten days you’ll have more Pikachu then there are protons in the visible universe.
Believe me, It’s way better if you know exactly why a child named Robert’)DROP TABLE STUDENTS;-- fucks up databases and that’s why the obscurity of some of Randal’s comics works so well. I often have no clue what he’s talking about but when I understand it exactly because it just happens to be something relevant to my work, the special feeling more than makes up for the ones I missed.
Naturally, when I’m bragging about being smart enough to understand XKCD, I spell [noparse]Robert’;)DROP TABLE STUDENTS;–[/noparse]'s name wrong. I’m obviously never going to break anyone’s database.
Well, sure. There’s plenty of XKCD comics that hit that spot for me on other topics. But my point was that you don’t need to know anything about SQL to find the comic funny.
The comic on equations for different branches of science requires a pretty strong knowledge of physics to appreciate.
Slate recently did an interview with him that’s pretty interesting.
His comics are all over the place, which is the appeal for me. Some require obscure knowledge, some just rely on an obscure observation that resonates with you, and some just trigger interesting emotions.
Some of his really interesting ones are those that used the web page medium for best effect. Two that come to mind:
Time, in which two people went exploring across the countryside. The comic advanced one frame every half-hour for hundreds of hours.
Click and Drag, in which you could use your mouse to drag the final panel around and explore an astonishingly massive world he had created.
Shit, I could go on all day. Here’s Dignified, one of his older ones.
In memory of Maggie the Ocelot, here’s Lanes. 
In fact, here’s the very first XKCD comic:
Feel free to click “NEXT>” (and don’t forget to always explore the mouse-over text) until you hit #2203 (the current one as of this post).
Even when I think I get a joke, there’s always much more to it, which is why I use an Explain XKCD Button. I always read the explanation.
But there were definitely some that I just totally didn’t get. Not many, but a few.
I started from comic number 1. Looks like this one, #5, is where he starts to get mathematical, and possibly “obscure” for those who don’t do math: