Especially when designing applications for mobile phones one must NOT forget usability. The screen size is definitely one challenge for a designer. How to fit enough information on the screen without making a mess out of it? Many suggestions may look great but from the usability point of view they just don’t work. No matter how marvelous your idea for an application would be, without carefully thinking about the UI’s usability it won’t be a success. The users communicate with each other! That shouldn’t be a surprise for anyone. And if good news travel fast, the bad news travel even faster.
See Tim Fidgeon’s article Design vs Usability.