Now that iPhone has been out for almost four months, there's a couple of potential features that Apple is missing the boat on.
iPhone should stream music via AirTunes to the Airport Express. You could stream your iTunes to any APX-connected speakers. It would be great to mobile DJ at parties, and if several people had iPhones you could trade off songs. It would be a cool way to share music with a group.
iPhone could also be able to be used as a remote control for things like a Mac mini or an Apple TV. If you've ever tried to use the horrible Front Row remote with a Mac mini media center, you know what I mean. While iPhone doesn't have Infrared (IR) built-in, Bluetooth would be more than adequate to control a Mac mini or WiFi an AppleTV. If you've ever used the Jonas Salling's excellent Clicker application for mobile phones, you know what I mean.
While we're at it, would it kill Apple to allow us to sync iPhones wirelessly? My old Sony Ericsson T610 could sync calendars and addresses over Bluetooth over iSync. And don't give us that stuff about Bluetooth being too slow - every iPhone has WiFi built-in. C'mon Apple, cables are so 1999.
What features do you think that iPhone needs the most?