Cloud service for itunes, so you dont have to keep your entire library on your phone. Download what you want to listen to and then delete when your done, kinda to replace playlists.
But there's many things needing changing, they haven't done much of anything new since iOS 4 came out almost a year again now, I really wish they wouldn't wait for an entire point update for any meaningful changes, notifications should have been fixed in 4.1
Or how about just hire some of the ultra-talented jailbreak devs to do it for them. Then maybe we can add new features, instead of getting nothing but a short battery life..
File manager. Seriously. It is needed. So badly.
Better notifications. Something between Lockinfo and OpenNotifier would be good.
Ability to change text/email tones.
I'd still use lockinfo + popup blocker, but it'd be nice for Apple to allow me to reply to a text without having to leave the app I'm in.
Ability to insert pictures in emails after you start typing instead of having to do it from the photos app, and a more refined version of Bite Sms where you can reply directly from the notification, without having to close your current app. Notifications that appeared on the top bar like Android would work great in addition to that