Prose-level mechanics are important. I'm a believer in the old saying, "There aren't really any new stories." Any story you write has been done before, so it's all in the execution. The word choices you make, and your ability to judge whether a scene calls for a leisurely, descriptive pace when describing character actions or whether it needs immediacy and brevity, is one of the things that will mark your skill as a writer and make your story interesting and readable.

However, prose-level mechanics aren't everything. The Time Traveller's Wife reads more like a "literary" novel than SF not so much because of the style of prose (plenty of SF/fantasy authors write in a "literary" style that would be recognized as such if their stories weren't about spaceships or dragons), but because the story isn't really SF. Time travel in Niffenegger's novel is just a MacGuffin. The story is about Henry's relationship with Clare, with time travel being the plot device that allows Niffenegger to tell the story in a non-linear fashion, and otherwise being quite incidental to the plot. The literary critics and readers who loved The Time Traveller's Wife would judge it very differently if Niffenegger had thrown in, say, a sub-plot with other time travelers, or scientists trying to figure out how Henry's ability works, etc.