While bestsellers often feature crappy language and elementary writing, they are nearly always tight on plot. They are full of breathless build and hard to put down; that's what makes them so popular with the masses.

You can do both, though--have beautiful language and a great plot that keeps people turning the page. And sometimes a book like that will become hugely successful. But very often, even though it doesn't top any charts, a book like that can achieve enough success to make the author a good living, especially if she publishes subsequent books of the same quality. (Incidentally, this is one of my main reasons for my love of speculative fiction. More attention is paid to plot than in literary fiction--it's often said that in science fiction and fantasy, you absolutely can't fall short with your plotting, since you are already asking the readers to suspend their disbelief. But at the same time, sff writers tend to not skimp on language like mega-bestselling authors often do.)