@allenthomas Glad this chat's been useful for you--it's the first one I've started, and judging from the replies, there are a lot of people who've thought a lot about this issue.
I asked this question because my comic, Starve the Beast began as a fantasy short story I published years ago. But when I came back to the idea, I realized that the central fantastical element (a dictator's beyond-death vengeance) worked a lot better with scientific influence. Basically, the country's a genetic engineer's Disneyland, and when the dictator in charge is murdered, all the research gets released.
A lot of the plagues (mutant plants, for example) aren't so far off from reality, so I thought keeping it scientifically grounded worked better. But because of my artist's style, things sort of take on a fantastical tone anyway (see below)
I think that no matter what genre you choose, rules are always important. Even my favorite fantasies have established world rules (even if they're very expansive).