Re-reading the Chronicles of Narnia. I probably read the whole series a dozen times over between 2nd grade and high school.
Little things just started jumping out at me, like if Aslan is the Christ analogy, but his name is Aslan, why the heck is everyone celebrating Christmas?
Several parts in several books also struck me as just ridiculously racist or sexist as well. I cringed my way through almost all of Last Battle.
Lucy used to be kinda cool, but now just strikes me as whiny.
I think one of the big reasons it has aged so poorly is that I wasn't really familiar with some of the things the books referenced. Puddleglum and the Green Witch's showdown is just a lot less exciting once you're read Plato's Allegory of the Cave and go, "oh, this isn't so much a battle as a short essay on why Lewis doesn't like Plato."
Far too many moments like that.
Voyage of the Dawn Treader probably held up the best, but even it seems kinda heavy handed as hell with its moralizing. Young Adult books have gotten a lot more subtle and far more relevant towards their actual audience. I mean, what second grader (when I first read and loved the books) is going to go, "Oh, I get it, the monkey named Shift is Darwinism!"