One of the things that keeps its simple, almost stark UI from looking amateurish to me is the quality of the animations. It’s not just that there is animation—that’s assumed for an iOS application—it’s that it has a particular character.
Take a look at the progress indicator. It’s a nonstandard shape, dots making up what looks kinda like a mostly-filled beaker, and it rotates to show network activity. But it doesn’t just rotate; it overshoots its rotation, and then settles back. And it doesn’t just turn in the same direction endlessly. Sometimes (but not always) it doubles back one.
These movements aren’t just a rote application of a standard iOS animation, a straight line from point A to point B. They look real, physical, like something alive, like it’s dancing.
The Macstories article on Letterpress (which has screenshots and videos of other cool animations in the app) also mentions that its style is similar to Microsoft’s new Metro user interface. Now, I haven’t used Metro or Windows 8. But if Windows has the same kind of sparseness to its visuals, does it also compensate for that with lively animations? It seems like it would need to, in order to engender the same positive reactions.
And woe to any developer saying, “Oh, I can do UI like that! It’s simple!” Any time saved with the appearance will just be eaten up trying to get the more complex animations to work.