What makes a phone an app phone is not the ability to run apps, or even that it’s a great app development platform. It is the fact that people are using apps in great and growing numbers on that phone, that there is a sense of gold rush among developers, that people are paying for apps and that there are huge hits.

By this measure the preeminent app phone today is the iPhone.

True, Android is massively successful in the market. But not primarily as the OS for an app phone. It is successful as the OS powering the majority of devices in a new category of Internet enabled phones that is devouring the feature phone market. For some time Android has represented the only way most manufacturers could create a phone with a decent web experience, so iPhone aside, it now completely dominates this new category, powering 80+ models.

The app phone might be a transient thing. HTML5, CSS, SVG and other Internet technologies, coupled with low latency 4G networks like LTE, might eliminate the need for rich client apps. But today there is no question that these apps offer a better user experience than their web based counterparts. The success of the App Store alone proves this point.

So given the importance of apps, I started wondering why Android phones aren’t app phones to the same extent as the iPhone? Where, as John Gruber asked recently, are the Android Killer Apps?

Much has been made of the reasons that developers like building iPhone apps, and I do think that Gruber is onto something when he points out the similarities between Apple’s mobile app platform and the successful game consoles. It mostly boils down to the fact that the App Store is a vibrant marketplace where apps are plentiful and so are the people buying them. But how did this virtuous cycle get started?

I’ve seen little to explain how Apple got mobile apps to cross the chasm to mainstream consumers in such a rapid and dramatic way. It is hard to remember this now, but prior to the iPhone the conventional wisdom said that apps would never be big on phones. Much less the runaway success of the App Store. Sure, Apple has the vertical business model to deliver a great end to end solution, but what, specifically, is it about their solution that made people comfortable running apps on phones?

I don’t claim to have a complete answer, but here are four aspects of the user experience that I think were important factors.

1. Everything, even the phone, is an app

Turn on the iPhone and the first, and only, thing you see is apps.

When the iPhone came out it was striking that everything, everything, was an app. Even the voice call functionality was encapsulated in an app. This was a massive departure from phones at the time, which all had send and end buttons. The mobile phone had been a physical thing and the iPhone made it a software app.

Put differently, the iPhone was a level playing field for all the areas of functionality, and the only way to play was to be an app.

The other important thing in this regard is what the iPhone still doesn’t have. Gadgets, or widgets, the small half-apps that don’t occupy the full screen but still do stuff. If iPhone had these it would detract from the purity of the everything-is-an-app model.

“But the iPhone didn’t have 3rd party apps on launch!” Irrelevant. Every piece first of party functionality was an app and the conceptual model was in place. The foundation was laid for the 3rd party app model that followed.

2. App as (only) entry point

Anyone who has thought about the conceptual model for a phone user interface, and in particular, the “shell”, will know that the relative importance of data versus app always becomes an issue.

For example, how do we want the user to find an email message, calendar appointment or song? Should we make apps the entry point, so the first thing I do is to find the relevant app? Or should we find a way to surface the individual email messages, calendar appointments or songs that I’m most likely to want next? For example, the latest unread email message, the next upcoming appointment and the song I listened to last.

This trade off led Microsoft designers to the home/start duality. The home screen is the topmost thing you see, and it gives you quick access to the things the phone thinks you want most. The start screen is one click away and gives you the list of apps. The intention is to offer the best of both worlds, with a little more emphasis to the data. It seems like an intelligent compromise.

Apple’s decision was: no compromise, apps are the only entry point. As a result the organizational model is an order of magnitude simpler. Turn on the phone, and you know what to do. The only thing you need to discover is that by swiping the screen you can access another page of app icons.

3. Icons as physical objects

Part of the iPhone’s everything-is-an-app model is the elevated importance of the app icon. The iPhone UI at its highest level is simply a canvas of all the app icons. I think this is enormously powerful for reasons that dig deep into our psychology. Essentially, we like to collect things. Physical things.

The visual design of the iPhone app icon communicates that it is like a little physical pebble. Unlike other platforms, iPhone icons have no negative space. Every icon has the same solid shape. They can optionally have a gloss, which is applied by UIkit and on by default for developers, and the uniformity of this lighting effect across many icons further reinforces the effect.

I feel like I could take all my apps and put them in a little bag and carry them around. Just like the collection of marbles I had as a kid. There is something viscerally satisfying about the physicality of those little app icons.

You may be thinking you’re too sophisticated to be influenced by this, but I’m pretty sure we humans haven’t evolved as fast as our technology over the last few decades. At a base, even unconscious level, we still have an affinity for physical objects. Apple’s design benefits from this, and maybe even exploits it intentionally.

4. The comfort of strict control

Once people are comfortable with the concept of running apps on a phone, you need to get them past the fear of installing something harmful, especially the majority of the market who use Windows and therefore have to be constantly vigilant about threats from malicious software.

The App Store completes the app phone solution. It is easy to use and it is trustworthy.

People who criticize the App Store based on its evil and restrictive closed-ness really miss the point completely. To a consumer who is nervous about apps, the restrictiveness of the App Store is pure genius. Apple represents a benevolent despot that decides what is and isn’t good for us. Their control sometimes irks us, but we love them for it.

We read about developer frustration, but who cares if the developers have to walk through fire if (1) the apps in the store are cool and, more importantly, (2) the apps in the store won’t kill my phone or steal my identity. We hear that some interesting app was rejected, but who cares about false negatives (nice apps that don’t make it in), if that also means there are no false positives (evil apps that do make it in).

Every time Apple competitors fueled the press about Apple’s tyrannical approach to app approval they were doing fantastic positive PR for the App Store. Making people more comfortable with the idea of downloading their first app, and making the openness of the Android marketplace ever more scary.

The ability to do “simple”

To me these aspects of the iPhone user experience illustrate Apple’s beliefs on just how simple things need to be before people will adopt something new. The iPhone shell is really stunning in its minimalism. But in order to get people over the adoption hurdle and into regular app usage, this is what Apple believed it needed to do.

This ability to do “simple”, down to the usability impact of negative space in an icon, is one of Apple’s core differentiators and it is incredibly hard for other companies to replicate, even with the worked example in their hands. In fact, if you are reading this and nodding your head, you might not really understand how truly simple new things need to be. If you are part of the 1% minority that does understand, then you still have to combat your own overwhelming desire to make that one tiny feature addition that tips the experience over the edge into complexity. And of course, you have to combat the 99% of your team or company that thinks the simple thing is dumb and not cool enough and won’t be competitive.

I don’t know the inner workings of Google’s Android team, but I spent years at Microsoft, most of it in the mobile group, and I shudder at the prospect of arguing for “everything is an app”, the app as only entry point, icons as physical objects or a strict and restrictive app marketplace. This isn’t a criticism of Microsoft as much as a comment on the fundamental DNA, culture, values, or whatever you want to call it, that makes Apple good at introducing new things to the world.

Evolving with the market

The iPhone won’t always be as simple as it was at launch. Apple will evolve the product along with the sophistication of the market. Already we can swipe left to search and bypass the app icon as entry point to our data. And we can imagine multiple apps running simultaneously with multi-tasking. The App Store has loosened up some of the restrictions and has introduced new models like in-app purchase.

One day, the iPhone might even have glanceable tiles on a home screen, or gadgets that display live data, or “hubs” to group app functionality. The App Store might be a free for all where we take responsibility for our own protection. But only when Apple thinks we’re ready.