We've talked about the tremendous opportunity presented by Apple's iPhone (and, soon, the T-Mobile Dream, powered by Google's Android!) for inspired entrepreneurs to design applications that can form the crux of a successful start-up. All well and good, you might say--but what makes for a successful iPhone (or smartphone) application?

An article in Forbes suggests that you go to school on Apple's putt, looking for the characteristics of the awesome applications that Apple itself has designed (Apple allows other companies to list applications in its store pending its approval, but has also designed some of its own apps, which are also for sale/download).

Where Apple doesn't distinguish itself especially, according to the article, is in sheer inventiveness: while many of its apps are far from duds, they don't particularly stand out in terms of a sheer "wow" factor from many of the apps not designed by Apple.

So what makes Apple's apps special? Unsurprisingly, the same thing that makes Apple's products special: usability; consumer-friendliness.

This is not something to be underrated. Even the savviest computer--which, frankly, is probably not a smartphone's main target demographic in the first place--values ease of use. As one senior research scientist tells Forbes, "Novelty increases people's tolerance for unusable things. As they use it and the novelty wears off, some of these usability problems will start to be more irritating."

The article points to Apple's clear visual cues informing users how to unlock their phones--that now-famous sliding bar--as a great example of how Apple makes using its phone as easy, simple, and stress-free as possible. It also mentions that on the keyboard that appears when one is typing a url into Safari, Apple's Web browser, on the iPhone, there is a ".com" option, thereby saving you four types.

It's the little things. When you're talking about something as ubiquitous in a person's day as a phone, those little things add up. Something to think about as you set about designing that killer app.