You can replace the built-in apps with different ones but this can go only go so far as to change the icons you tap on dashboard. The lack of unified way for apps to interact with each other is seriously crippling the platform's capabilities.

As I use both an iPad and Android phone on daily basis, I'm find this especially lacking in tablet setting. An app wants to open a web page in standalone browser? They will always do that in Safari. Sending an email? Only through the default app. About the only 3rd party application that I can consistently use from more than one place is probably Read It Later (called Pocket now) - and that's only because apps' developers took time specifically to integrate with it.

And that's basically how it works on iOS now: for application X to interact with Y, it must specifically account for Y. Any similar replacement app Y' or Y'' will not be supported unless handled separately. It's easy to see how this model doesn't scale to diversity.