Examples

• No single help index
• Applications don't see the same URI space
• Drag and drop, cut and paste don't work when they should
• MIME associations are different per-app
• Sysadmins have to figure out a different config setup per-framework
• ISVs can't install menu items and MIME associations in a single way

Not everything can be addressed on GNOME/KDE level; for example, X server enhancements. Another need for a real base platform project/community.