Usually you would only want to backup the files and databases that are important to you or irreplacable (without a lot of work).
Operating systems can be reloaded from CD/DVD in the event of a hardware failure.
Examples of files and file types you would probably not need to backup
- Operating System Files (excluding certain Servers with Active Directory)
- Program Files (unless the actual data for these programs is stored within their Program File directory)
Applications can be reinstalled, but the data they make cannot be remade.
- Data already in the cloud. Data such as your iTunes purchases can be redownloaded. You do not need to spend bandwidth and money on storage backing up something that is easily accessible from anywhere already. This is not just for music.
- Game Installations
The game can be reinstalled. However, make sure you backup your saved games!