Better email client. We’re on the sixth generation of Apple’s mobile software and I still can’t drill down my inbox view so it only displays unread emails. I also still can’t mark an email as read without opening it or navigate to my sent items without backing out to the main account list, scrolling down past the inboxes and navigating through the main folders. Come on.
I probably spend 75% of my time on my iPhone reading and writing emails, and yet Apple's mail app is behind others on the market, especially the now-Google owned Sparrow.
Aside from the problems BGR mentions, I hate how the app forgets the email address of people I have exchanged hundreds of emails with, just because none of those emails were in the last few hundred sent or received. Even Google's web-based solution is better than that on the iPhone.