That right there is, to me, the biggest miss in iOS and why I am quite ready to move to Android entirely: it is very convenient to be able to share data between apps. Take a picture with the camera, share it out of the gallery into an editor, then just the same share it out of the editor into an SMS, Twitter status, an Evernote, or anything else that registered itself for that filetype. I love it so much I actually miss the functionality on all the desktop operating systems. iOS, by comparison, forces one to use either the predefined list of sharing services, or to keep saving things and opening them in each app one by one.
I can see that Apple has noticed the omission, judging by the 'arrow out of the square' share buttons showing up everywhere from desktop Safari to Mail.app, but it's still very limited and by comparison.
However, the number one feature I wish I had on my iPhone is the Android intents system. It is seriously great. I'd love to be able to set the default application for different actions. When I tell my Nexus 7 to play a particular song it will now launch Spotify to play the song I requested. I should be able to do this with Siri. This would be a huge feature for me.