I felt for all of his characters deeply, and I thought the story was told brilliantly. Particularly striking was his account of a murder from the perspective of a child: how unfamiliar things might sound like familiar things out of context. He has a real sense of how trauma can impact and stay with people - children and adults - for years and years, something that's often missing from thriller & cozy murder mysteries.
Additionally, Sullivan's writing is fun: he uses surprising verbs and adjectives, or uses them in surprising ways. Many sentences have this delightful, lively quality. I'll be looking for more of Sullivan's work.