From my non-expert point of view, my distinguishing criteria between a novel and a series would be the same. If you're writing something more easily digested, you can give a reasonable hint in a one-liner, genre, and tags to give an idea if something's someone's bag. With something that requires a major investment in time for all involved, including a potential reader, you'd probably need a synopsis feature, meaning, again, extra resources for one aspect that other styles don't need. It's a lot of accommodating for a small number of pieces of creativity; I think that's the point I'm trying to make, even if I'm doing so badly!
