I've realised that science fiction and fantasy both work best when they borrow something of the approach and attitudes of each other. Fantasy is very good at triggering subconscious reactions in the reader. All those mysterious, magical, mythical elements resonate strongly with the subconscious. Science fiction on the other hand appeals to the reasoning, conscious mind - raising philosophical questions about the nature of reality, science, time etc However, I'm starting to learn that both of these genres can get carried away with themselves if you're not careful. Science fiction can get unbearably weighed down by an obsession with sticking to rigorous scientific fact, until all story and wonder is sucked out of the thing. Fantasy can get ridiculously overblown and moralistic to the point where it just tries to be too big and also ends up tiresome and lifeless. The solution is simple. Let each one adopt a little pinch of the other. Provide a rational framework to your fantasy stories. Make sure magic has rules, real rules that function like a physical, scientific fact. Give some kind of rationale for what magic is and where it comes from, as if it were a scientific reality (although the explanation can be fairly magical and otherworldly). Try and stretch beyond concepts of good and evil, making the morality of the tales more complex and ambiguous than that. Suck as much spirituality out of the genre as possible to avoid preaching. It'll still be fantasy because it will still contain all those mythical elements that trigger strong subconscious responses. But it won't be fantasy that gets all overblown, preachy and carried away with itself. Create atmosphere, even spookiness, in your science fiction stories. Let the subconscious triggers that fantasy does so well come to the fore. Focus on characters and situations rather than grand concepts and theoretical science. It'll still be science fiction because it will still have robots, spaceships etc and it will still have a scientific rationale behind what's going on (yes, and hopefully accurate scientific explanations for things) - but it will be magical, awe-inspiring science fiction rather than dry and geeky dross. Just some insights I am having recently. I am aware that what I need to do might not be relevant for others. But I wondered what other people think.
As do I, some of these points were brought up in some of Limyaael's rants (find links in General Writing stickies). Also, I've always made my magic systems with limits simply due to the fact that I HATE deus ex machina magic.