I never did. I couldn't quite comprehend the reasons behind that argument then, and do not do so now. Personally I believe in giving something a shot before dismissing it totally. I mean sure, you can call a panda character childish, but does it really mean it's true? The whole Kung Fu Panda thing I found a bit ridiculous.
Having played some (real life permits very little - that's why I've only leveled my main thus far and she's nowhere near done with reputations), I still don't get the argument. Sure, there are jokes, puns and "fun stuff", but that doesn't make it any more childish than any other expansion. All expansions have those moments. I like the fact that it's not all so serious from the beginning, and provided that Blizz can forward the story well in patches to come, I guess the story will take a darker turn soon enough as it is.