Nothing
I'll keep this simple. I like work that doesn't try too hard. It's so easy to work on a project until I've beaten every last bit of life from it. It's good to know when to stop. And the work I like best looks like the designer did one thing like set the type in Akzidenz Grotesk and then said, "Yeah, I'm done." Perfect.
Young designers tell me all the time, "Are you sure, it seems empty." But the idea makes it full, and in fact it's not empty, it's filled with a ton of negative space. I think of it like dark energy and dark matter. It's strong enough to hold everything together. I deeply covet Richard Danne's desk calendar from 1974. I think there's that place in hell that I've mentioned before (the one where amateur musicians pull a guitar out at a party) for people who steal. But, I'd steal it.
All of these projects are confident and clear. They resonate with harmony because every tiny detail has been refined, refined, and refined. So try this on your next project. Do one thing and stop. It'll be hard and the evil workings of layers in Photoshop or Illustrator will be calling, "Add more, add more." Resist.