How about a decent implementation of Meta-Data, like BeOS/BFS/Tracker. This could get a bit complicated, but would definately made up for it for Power-Users. This would also require a live Metadata Updater, which extracted ID3-Tags, JPEG EXIF Data and Word Document Attributes to FileSystem Attributes on-the-fly.
Other Things that would be nice are updates for iSync, AddressBook and iCal, so they can have better support for existing devices, and an increase in the number of devices supported. iSync still feels like a Work-In-Progress.
Better Image Support is also a High Priority for me too.
The Reason PNGs are so fantastic (IMO), is Adam-7 Interlacing. When Progressive JPEGs and Interlaced PNGs don't show up any differently to Standard JPEGs and non-Interlaced PNGs, it's quite detracting. Nothing worse than having to wait for a large PNG to finish loading before you know what the hell it's supposed to be.