Subversive in that the purpose of art (and therefore fiction - in my opinion, I'm not an art major or philosopher, just a guy who sometimes sputters around his blog and collection of WIP word docs) is to engage the reader at a level that subverts our often cautious and critical intellectual barriers. Barriers that sometimes blind us to truth and new perspectives.
For the rut-riders (whether liberal or conservative) art can be uncomfortable. Dangerous even. And if fiction is suspect, then speculative fiction even more so. Speculative writing (science fiction, fantasy, etc.), by definition, broadens the boundaries of what is/can be and therefore is not safe for the casual reader.
One person commented/joked here recently that we Christian readers/writers of SF are suspect ('going to hell, right?') among our conservative brethren. And I agree. Some readers don't want their fiction to be art, they want it to be a treatise reflecting their own beliefs, values, and worldview.
Again, this goes for both liberal as well as conservative readers, but let me give an example from the camp I'm most familiar with. In Christian publishing there still seems to be a stigma against speculative fiction. And there is a prevalence of "bonnet books." Why? Because these books are safe, conservative (can't get more conservative than an Amish woman in a bonnet!), and although it's fiction, readers believe it to be more realistic and historically grounded. I know that's a big assumption!
If I'm right in that speculative fiction broadens the boundaries of what is/can be, then it can't be safe for the reader. It is moving toward art and away from treatise. There's more I want to say here, but have to run. For now, the point for me personally: I want to write spec fic that is thematically engaging, but not necessarily religiously accurate. I don't want my art/craft to be a doctrinal statement, I want it to touch the reader and seep into a person's soul and not simply entertain the mind.
I want to be subversive.