Storytelling, like most arts, has a way of revealing more about the artist than it does about the story being told. How characters are portrayed, the plots and events the author chooses to tell, how the exposition is handled: all these and more lie shallow in the narrative like gold glitter in a stream bed, waiting for the patient reader to patiently sieve them out and hold them up to the light. There’s no rule that says you (as a reader) have to do this, of course, and many times you won’t feel the need. Sometimes, though, the artist isn’t content to let the gold wait patiently for you to find it. No, sometimes they want to grab it and shove it in your face.
This is where the art drains away from storytelling, to be replaced with something else. When you hammer home a point at the expense of the story, you’re no longer an artist nor an advocate; you’ve become a demagogue, and if you’re going to do that, stop pretending to write fiction and start whipping out tracts. There’ve been plenty of offenders, at varying levels of skill. Ayn Rand leaps right to mind, and so does Tim LaHaye. Both of these folks used fiction to get across their pet hobbyhorses, and in both cases, their fiction suffered. I managed to read Rand at a time when I was receptive to her ideas, but not receptive enough to read anything other than her two big novels. As far as LaHaye goes, one book was more than enough for me.
The trick here is to give your hobbyhorses enough rein to go where they want to go, yet not let them overcome the story. Although I’m not a fan, C.S. Lewis managed this trick fairly well with the Narnia series. He had a point to make beyond the fantasy tropes he used, yet he did it skillfully enough that you don’t feel overwhelmed if you’re not into that. To pick more modern examples, I’ve always admired Orson Scott Card for this ability. His fiction covers a lot of philosophical ground, and despite certain strongly held beliefs (some of which I’m personally not too keen on, particularly as expressed in an infamous essay a few years ago), he manages to write stories in which neither the presence nor absence of his beliefs figures, even if the topics arise in the stories. To put it more bluntly, he lets the stories be what they are, and (at least, in my opinion) doesn’t force them to carry his ideology.
On the flip side of this, you have artists like Frank Miller, whose hard-line political beliefs have caused some of his more recent work to curdle around the edges. Miller, renowned for his dystopic and noir visions best realized in The Dark Knight Returns and Sin City respectively, has let his art become the soapbox for his increasingly bleak and hawkish worldview. Ever wanted to see Batman take on Al-Qaida? Miller’s working on it, in a graphic novel tentatively titled Holy Terror, Batman! that the creator has acknowledged as propaganda. I’m not sure which would be a greater disconnect: the “I’m the goddamned Batman” stylings of Miller’s All-Star Batman run or a wartime propaganda piece starring the Dark Knight. Then again, that’s the point. Miller is undeniably talented, but he’s letting his hobbyhorses run wild with his work.
Good news, though: Even if you do let your pet obsessions take over your work, you can always take it back once you know which beasts are running through the paddock. Dean Koontz managed it; for a while, all his books seemed to be sliding down a greased pole into apocalyptic fever dreams, culminating in his horror-genre Rapture redux The Taking. However, with books like Life Expectancy and the Odd Thomas series (even though they usually feature big bad events), Koontz has demonstrated the willingness and ability to lighten up a little. More than anything, that’s the prime thing to remember if you’re going to make your story mean something beyond its purely narrative elements: keep your thematic brushstrokes light, and don’t let your story suffer. Readers will give you a whole lot of slack if you’re skillful. Don’t abuse that.