No matter what visual style is hot, no matter what great product you're using, every piece of good design uses "white space" well. I'm using this as a blanket term, not only to mean "negative space," (or places where there just isn't any stuff, like typography or imagery), but also to describe other invisible relationships between elements of a design, such as their proportional relationships, and the way they line up with one another.
This is truly what matters in creating that "clean" look: being able to use these subtle relationships between things on your design to communicate their hierarchy and how the pieces of information relate to one another. Someone just learning to master visual design could learn a lot by ignoring everything else and focusing on mastering just white space. I believe this so strongly that I've developed an entire course on white space that will be available to my Design for Hackers email list soon. In the meantime on Hack Design, I'll piece together the best things I've found around the web to help you see the unseen.