iPhone OS is not even 3 years old. You can't compare it to Windows Mobile 6.5 or Blackberry OS, because they've been around so long and have had so much time to add all the features they have.
Originally Posted by DBlanco
Even with younger smartphone OSes: Compare iPhone OS to Android. There are less individual features built into the firmware, but iPhone OS runs much better. Why? Because Android was built for all of its main functionality at once, and then a UI was slapped on.
With iPhone OS, Apple started with the UI. It's the single most important part of iPhone OS, and it's what set it apart from the competition (until now, Windows Phone 7 has a great UI and webOS is great too). They built the major functions slowly to ensure the highest quality performance and hardware optimization to provide the best user experience. Look at WP7, it's completely bare in terms of features aside from some sweet MS office functions. It has no copy and paste, no multitasking (despite what the fanboys will tell you), no customization, etc..