@ Jack Spencer Jr.:
I somewhat see where you are coming from, but, I very much disagree. Those two comics perfectly illustrate why CAD is crap PA isn't.
The CAD comic is over-explained, drawn-out, mistimed and very, very, very poorly drawn. As Plowking points out, he explains the joke in panel 1, then goes on to make said joke. Why? You've just explained it...

Let me try to put this into joke-telling perspective....
Joke: Punctuation. It's the difference between helping your uncle, Jack, off a horse and helping your uncle jack off a horse.
CAD Joke: You know how there's like, punctuation? Yes? And you know how if you change it around, it sometimes makes a sentence mean something else? Right, well that can be really funny! Really! Like, for instance, Jack, is a name, right? But it can also be a verb, when combined with off *sniggers* and it means a dirty then! Bwahaha! So you could say "help your uncle jack off a horse" instead of "help your uncle, Jack, off a horse"! HAHAHAHAHA!
I hope the difference is obvious...
And then, as said, there's the artwork. Or rather, there is in the PA comic, and there isn't in the CAD one. Now, a comic doesn't always need good artwork. Drawings can be used simply as a background for a joke. But then, your joke damnwell beter be good. Tim fails there. Epicly. In both comics the joke isn't very clever, but, PA makes something of it by combining it with some solid artwork.
Should a joke never need to be explained... I don't know, I think sometimes you can get away with a bit of explanation, but it's an art to do this without ruining the joke. Should everyone be able to get a joke, regardless of their knowledge of the subject? Hell no. If I decide to dedicate a comic to the antics of character in a videogame, I'd be writing this mostly for people who know said game. Sure, You can probably make a couple of jokes that everyone will get and laugh at, but "in-crowd" jokes will in the long run be inevitable. Also, they can be very, very funny, though not for everyone.