Richness of text is important, I think, to pull a reader back in. Story, plot, etc., are of course very important, but once you know the story -- even know it in detail -- what brings you back to a book over time? The writing. The richness of the prose, the way the writer uses that prose to refashion the world so that you see it differently. It becomes like a multifaceted gem, which can be used as a prism: turn it this way, you see this aspect of the world; turn another, you see something else, and so on. Or like a fine wine, with subtleties that tickle the tongue and palate (and mind); something that can pull you up short and with a catch in your breath... sometimes even after you've read a piece many times over. All too often this is seen as "purple prose" rather than simply "good, meaty prose".
Again, I think this has to do with our particular period in history; while more people are literate, less people are reading deeply or critically; therefore simple, shallow prose is enough, as they want the story, and seldom read a book again. It's like the difference between looking at a sketch for a sculpture and the finished sculpture. One is informative and certainly takes craft and skill; the other takes art and breathes life into the subject. Too many writers of the 20th century have fallen into the former category, where there's a glibness and surface sparkle, but not much depth.
As for use of irony... anyone up for some John Collier?.....