I agree with Saanen.
I think the most important things for me as far as a good fantasy/sf book are originality, believability and thoroughness.
Originality: I want to read about new worlds, not rehashed old ones. I can't be bothered with stories that just recycle the old tropes and standbys. Elves, orcs, evil overlords, even dragons. If there's a new twist or facet that is explored - fine. But too many new writers just want to use the same old stuff that's been done so many times before.
Believability: The writer has created a world AND shows it in such a way that I half believe it. Not BELIEVE it, of course, but the suspense of disbelief comes easily. I enter the world the author has created and am borne through it on their words alone. It's a fun ride and while I'm on it, that particular world is real to me.
Thoroughness: I love it when an author has covered all the bases. I don't stop in the middle of a scene and think: "Why?"
The author has constructed a world, society and characters that act naturally within their framework and also interact naturally. The author makes me feel like there's an actual world still out there.
Bad stories have gaping holes in logic, in the world's infrastructure, society etc.
Some pet peeves: When an author creates a society of drones or clones. Everyone in X society is a stable, hardworking upright citizen and everyone in Y society is no-good, evil etc.
When the characters are unbelievable - as in goody goody's with no bad qualities, no errors in judgement, who show none of the baser human qualities that every human being on this planet was born with.
Yes, I want heroes but I want them to be believeable and find it more interesting when they perform heroic feats despite the fact that they sometimes feel lust or jealousy or greed or whatever.
I have to say that I think the face of fantasy today is changing. 5, 10 years ago I felt like fantasy was in a la-la land with all the cookie-cutter hero archetypes, the connect-the-dots storylines, the same old, same old cast of characters, villains and monsters.