I prefer writers who use clever turns of phrase. I love Terry Pratchett. I also prefer more concise writing styles, if the writer is too long winded I end up skimming even if I like the story. I also enjoy more lyrical or evocative styles, I like beautiful language. I like a writer who demonstrates skill not just ideas and lofty intentions. As in poetry, it is better when the poet puts more thought into it than just pouring out their feelings in a cathartic rush.
In non-fiction, I prefer well constructed arguments with an engaging style. I get irritated if I have to wade through tedious minutia. I want there to be enough explanation/examples to understand the point, but I don't want to go through every possible example. I enjoyed Brian Green's The Elegant Universe.
I do not like writers who needlessly complicate their sentences or arguments. If you can't say it concisely, you haven't thought through it or edited it enough. Even though my own style tends to ramble too much; I always end up over clarifying and muddying things.
I do find style important, but it is not the most important part of my enjoyment of a book. So long as it isn't terrible. Perhaps a 6 or 7 on a 1-10 scale. I read a lot of amateur stuff online and some of it has a good premise, but is just so terribly written that I can't get past the first couple paragraphs. Though, I have also read some things from an editor that I just keep rereading to enjoy her absolutely wicked and witty style. I do find myself reading things I wouldn't have normally if I really enjoyed the writer's style in a previously read piece.