In most cases it's the critical files that need to be backed up.  These would be the files that you can not reproduce or would require a tremendous amount of hard work.  Below is a list of different things you may want to consider backing up.

Personal Home Use:

• Email Files (i.e. Outlook PST files)
• Bookmarks / Favorites
• Music Files
• Banking Documents
• Family Photos
• Home Videos

Work Use:

• Sales Proposals
• Meeting Notes
• Email Files (i.e. Outlook PST files)
• Bookmarks / Favorites
• Excel Spreadsheets (i.e. *.xls, *.xlsx)

Server Use:

• Client Data
• Application Configuration Files (i.e. *.ini)
• Exported Registry Files (i.e. *.reg)