The simplest way to draw the line is to not draw it at all.
If we try to say "Ents bad, Kzinti good," then we're going to just get into a million arguments over where to put Tribbles.
If we try to differentiate with tags, then we're going to get into tag edit wars.
So, here's my idea: let's just… not.
If we have to have some differentiation of what is/isn't on topic, let's think about it the way the Hugos do (emphasis mine):
While the World Science Fiction Society sponsors the Hugos, they are not limited to SF. Works of fantasy or horror are eligible if the members of the Worldcon think they are eligible.
and also here:
anything published in the correct year that the voters think is either science fiction or fantasy.
Think about it this way: do we really want to try to say (as just a single example) that questions about the World Science Fiction Society's 2009 Best Novel winner are off-topic?
Anything the community wants to ask about here is fine. Let's see what the community itself finds interesting.