I still have a soft spot in my heart for Narnia, but I was aware of their allegorical underpinnings from the time I first read them (having been raised in an evangelical Christian family, it was considered a feature of course). Even though I've shifted away from many of the assumptions embedded in the Narnia books, the nostalgia seems enough to sustain my enjoyment these days (and hey, they are well-written children's lit). I can appreciate what a big turnoff it would be to discover the allegory afterwards. That strikes me as creepy, like realizing someone only befriended you in order to proselytize to you.
My story turnoffs are:
- Anything overly sentimental, in a sickly-sweet sort of way, unless it's really well executed. I don't mind being emotionally manipulated, but the moment I see the strings, I get annoyed. (Example: pretty much everything Mike Resnick has ever written. Ken Liu occasionally falls on the wrong side of the line for me, but I usually tend to enjoy Ken's sentimental stuff because his execution is so good that it doesn't feel cheap.)
- Faithful Pet (esp dog and/or cat) stories, particularly when the pet dies at the end, a la Old Yeller. Okay, it's basically a sub-sub-genre of "sentimental", but this one really annoys me because it's usually SO blatantly cheap and manipulative. (Example: I'm thinking of a couple Resnick stories again...)
- Male power fantasies. This would be anything that reminds me vaguely of a video game plot, where the point seems to be setting up obstacles just so we can admire what a badass Mr. Manly Muscle Man is as he punches, shoots, and karate-chops through anything that stands in his way. (Example: Conan stories, and some golden-age SF)