Sci-fi and fantasy often get put together in categories because, although one tends to be forward looking and technology-focused and the other is often backward-looking and magic-focused, they’ve both got great scope for imagination and stretching the bounds of possibility.
There’s a natural challenge in writing fantasy or sci-fi. If you’re aiming to have a world highly similar to reality or its past then you can write I, Claudius in space, if you wish. But if you’re going for something dramatically different there’s a tension between writing something realistic in a fantasy-based context and something which is so unorthodox and strange as to be ridiculous.
Magic’s a good example. There’s no natural boundary on magic because it doesn’t exist. So, what’s to stop an evil sorcerer snapped his fingers and brainwashing everybody into serfdom? Or a good sorcerer wiggling his ears and turning all evildoers into garden gnomes? Hence, the author needs to create a limit.
Another potential downfall is one which affects all types of writing. Making heroes too virtuous and villains too vile just makes them two-dimensional caricatures. When writing my own stuff, I’ve modelled the mages (who play a significant role) as a light version of the Greek gods. They’re arrogant, generally, and don’t mind abusing their powers somewhat when they feel like it. After all, if you could click your fingers and unclasp a woman’s bra, wouldn’t you?
A good example of balancing a very unearthly fantasy world and the natural frailties and strengths people possess is the Night Angel Trilogy, by Brent Weeks (I’ve read books 1 and 2, but not yet 3). There are differing schools of magic, a powerful criminal underworld, a number of distinctly different nations and fantastically strange creatures which I won’t spoil.
I think this is a problem Star Trek: TNG suffers from, unlike DS9. I cannot believe that we will ever get rid of money, or that hunger will ever be abolished or that the world will become everyone’s oyster. People just aren’t that good. In DS9, some interesting issues were tackled, such as treachery and insurrection/terrorism.
People will suspend their disbelief for magic and warp speed, but not for everything.