This is surprising only because the stuff on the site is freakin' hilarious! It's hard to find good cartoons nowadays that aren't oh-so-squeaky-clean educational stuff or oh-so-politically-incorrect-suck-on-that-b
The most potentially offensive stuff on the site: Strong Bad's aforementioned bad attitude (which, if you're teaching your kid the difference between right and wrong like you're sposta, shouldn't be a problem); occasional references to what is apparently beer, but since the word "beer" is never used (it's referred to as "cold ones"), it could just as easily be root beer... hmm, that's all I can think of.