Difficult Features / Enhancements:

StatusDesc.
Ongoing EXIF / IPTC support (including auto rotation of images on import, and extracting photo descriptions from the comment field tag)
Use KDE Native file dialogs and Roll my own file dialogs (for other OS's) (add photos / export / save ) or make better use of native file dialogs and button icons. Benno may have advice/expertiece here.
Import from cameras
Must Have Full screen slideshows
Export CD/DVD slide shows
Backup to CDR/DVDR
Accelerate display/effects using openGL and/or CoreImage
Must Have Sophisticated printing system
Must Have Email support
Investigate switching over to photo library approach other file types (movies, pdf's, etc)
Make slideshow + thumbnail image resolutions theme specific
Panorama construction tool
Add reeditable calender construction tool
Add reeditable card (ie greeting card + text overlays) tool
Must Have Additional artistic manipulations (woven photograph, Van Gogh painting effect (a la stary night), pointillism (Seurat), charcoal, sketching, water color)
Integrated spellchecker

Moderate Features / Enhancements:

StatusDesc.
Must Have Folder import
Must Have Easy theme/translation install/update without root priveldges
Ftp/sftp posting within program
Drag-n-drop between albums
Make histograms respond to each other and brigthness/contrast controls in histogram view
Switch to non-padded slideshow images
Allow users to delete photos within the editing interface

Simple Features / Enhancements:

StatusDesc.
Must Have Add elipses (...) to Album Statistics and Settings menu entries
Must Have Better save warning, use "Save changes before XXXX" and "Don't Save", 'Cancel", and (default selected) "Save..." buttons
Must Have Properly translate Ctrl, Home, End, Shift, Del, and Print keys thoughout program, (mainly help system)
Add option for resizing images when exporting
Add keybindings for moving photos around with collections, possibly even between collections. Ths is easyfor the trivial case (Ctrl + ← / →), but handling discontinious selections correctly will be tricky.
Add support for displaying a photos original file name