Here's a fabulous article by Nick Lowe explaining the mechanics of bad, but successful, writing. Very funny, very informative, and apparently he feels about the same way about the Thomas Covenant series that I do.
In all seriousness, part of the problem of attempting to write and sell an SF novel, is that many, or even most, publishers don't want the truly original, but at the same time, they don't want hackneyed. So you have to use all the tried and true plot devices and character types that read as SF, or you're likely to be rejected (esp. as a first time author), but you have to use them in disguised, or novel ways. A hard balancing act to manage.
Of course, this is one of the reasons I get so annoyed at successful SF or Fantasy writers who rest on their laurels, turning out more books of the same, instead of trying to really improve their writing and plotting. My classical example is Mercedes Lackey. Don't get me wrong, she's a good solid writer, and she's done some pretty fabulous books, but she rarely pushes herself. Her emphasis is on quantity of saleable books, rather than on the quality of her books. This is perfectly understandable from an individual author's standpoint - after all she has to make a living - but I think it damages, or at least signally fails to help, the standing of the genre as a whole. It annoys me sufficiently that I simply don't buy any of her new books unless I've read them from the library first, and know that I like them enough to reread them.
There's a number of authors that fall into this category for me. In fact, it's much rarer to find an author who's consistently willing to push herself and her craft rather than turning out more of the same over and over again.
Maybe I should thank them, after all, it's likely to make it easier for me to get published, as the standards are not as high as they could (or should) be. But I'd rather have a plethora of fabulous new books to aspire to, instead of trying to make mine hit exactly the right amount of hackitude.