With the iPhone, many developers assume (with good reason) that users will only have a minute to use their application — while waiting in a grocery store line, someone is setting up a presentation at a meeting, before class, et cetera. This emphasizes simplicity, ease of use, and speed over features.
In an article published today, Fraser Speirs points out that there are different mobile contexts than this, because the iPhone is increasingly being used as a notebook replacement rather than just a satellite device.