Syncing. Syncing should be pulled out of iTunes and made a core feature of the operating system.

Support for over-the-air syncing of devices.

DLNA. I use MediaLink to stream from my MacBook Pro to the Sony PS3.

My USB scanner doesn't work when plugged into the Airport Extreme.

Basically, everything related to syncing, sharing, streaming, and networking in general should be improved.

The future of the Mac is connecting to the iPhone, iPad, AppleTV, MobileMe, and other devices.