1. Excellent writing. This is everything from the mechanics to the diction to the voice. Do the Irish keep their language centers and music centers in the same part of the brain? Do they just use words better than everyone else? I particularly noticed the voice in The Likeness. How many sophomore novels are actually stronger than the freshman? Here's one, and Tana French had already reached a lofty height with In the Woods.
2. Excellent mystery, and plenty of it. These books are layered with mysteries. The mystery that drives the plot, and the mysteries that drive the characters. It's not just the one straight line through the book. There are complexities.
3. Mystery survives! These novels do not end with everything solved neatly, boxed, and tied up with string. Mysteries go on. There is more to know. Do not think for a moment that this is unsatisfying; instead it makes me hope that Tana French is a fast writer. I'm ready for #3.