On my computer, I can set up a default browser and mail app. Why can't I do that on my iPhone?
Thanks to the great variety of mobile apps I think that default app integration on mobile devices is even more important than on the computer. Here's in particular what I'd like to see:
Please, Apple, don't force me to use your apps. If the EU didn't let Microsoft offer Windows with their media player and also forced Microsoft to have every user choose their default browser, why is it fine that I'm forced to use Apple's apps on my iPhone? While I think that the EU went way too far with trying to get Microsoft release Windows without a media player and without a web browser, I think the missing ability to choose default apps on mobile devices is far more inconvenient for end users.
You have to remember that only in the last 1 or 2 years have we seen actual replacements for Safari and Mail. In terms of the Phone app, you're never going to see a replacement. That's most certainly a requirement of your carrier, to have calls routed through them so you pay for minutes. Same thing with Messages, no carriers are going to want a phone on their network that lets people easily send free texts because SMS plans have such huge profit margins.
One thing I do agree with you on is the inability to change to a default music app. Even worse, when I get in my car and its Bluetooth receiver pairs with my phone, it (almost) always starts a song from the Music app even if I was playing Soundcloud or Spotify not moments before. That's annoying, but it's definitely an encouragement from Apple to use iTunes and the App Store for buying and browsing music. Considering that the iTunes store is just a giant advertisement for the iPhone and iPod, I don't think it would hurt THAT much if they allowed us to choose a default music app.