Uh, really? I haven't read it in a while, but going back the last ten comics...
1489 - Physics joke that I get enough from context to get the joke (though not find it funny) even though I only really know the names of the forces beforehand.
1488 - Flowchart joke
1487 - Joke about Tornado hitting a merry go round being fun
1486 - Haha Vacuum has TWO meanings
1485 - Hahah OK this one is pretty great just
go read it yourself. No a priori knowledge needed to get the joke (maybe only funny to native English speakers / Americans?)
1484 - Knowledge needed to get the joke is at the header of the comic. Wordswordswords but pretty funny.
1483 - Ha, ha, language changes!
1482 - I guess if you don't know that music is comprised of notes ... or I guess don't know note names, this would be obtuse?
1481 - Point to Fenrir, I don't understand any of the API gobbledy gook, but I'd argue that that is the joke itself.
1480 - Hipster anti-anti-SuperBowl comic. Americans only I guess.
1479 - Ha ha, computers do stupid things.
1478 - Point to Fenrir
Yeah some of them are obtuse but most of them have jokes that are pretty easy to understand even if you're not knowledgeable in the field they're drawing from. I'd argue that the bigger problem with XKCD is that except for the one I linked none of them are really that funny.