The idea of making a backup is to back up as much as possible on your system, but some exceptions do exist as shown below. It is not logical to include these in your backup at the cost of time and space in your media for nothing.
The major exceptions to not include in your backup are:
The /proc file system: since it only contains data that the kernel generates automatically, it is never a good idea to back it up.
The /mnt file system, because it is where you mount your removable media like CD-ROM, floppy disk and other.
The backup directory or media where you have placed your backup files, such as a tape, CD-ROM, NFS mounted file system, remote/local directory or other kind of media.
Software that can be easily reinstalled, though they may have configuration files that are important to back up, lest you do all the work to configure them all over again. I will recommend putting them. the configuration files for software on the floppy disk.