Originally Posted By: tacit
Time Machine backups are a special animal. They don't back up operating-sysem-level files and aren't intended to be bootable, and they're more database entities than file-level entities.

The “aren't intended to be bootable” part is true enough (in the immediate sense), and easily overcome by installing an OS on the backup disk (assuming we're not talking about a NAS, which wouldn't be bootable with any backup software). However, the backups Time Machine produces certainly are intended to be bootable —once restored back to some disk.

Therefore, the “don't back up operating-sysem-level files” part may be a bit overstated. [or, i'm not sure exactly what you meant.] In either event, stuff that gets skipped by TM is generally considered to be expendable (though a few exceptions may be argued by some users).

$ defaults read /System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions
Code:
{
    ContentsExcluded =     (
        "/Volumes",
        "/Network",
        "/automount",
        "/.vol",
        "/tmp",
        "/cores",
        "/private/tmp",
        "/private/Network",
        "/private/tftpboot",
        "/private/var/automount",
        "/private/var/folders",
        "/private/var/run",
        "/private/var/tmp",
        "/private/var/vm",
        "/private/var/db/dhcpclient",
        "/private/var/db/fseventsd",
        "/Library/Caches",
        "/Library/Logs",
        "/System/Library/Caches",
        "/System/Library/Extensions/Caches"
    );
    FileContentsExcluded =     (
        "/private/var/log",
        "/private/var/spool/cups",
        "/private/var/spool/fax",
        "/private/var/spool/uucp"
    );
    PathsExcluded =     (
        "/.DocumentRevisions-V100",
        "/.MobileBackups",
        "/MobileBackups.trash",
        "/.MobileBackups.trash",
        "/.Spotlight-V100",
        "/.Trashes",
        "/.fseventsd",
        "/.hotfiles.btree",
        "/Backups.backupdb",
        "/Desktop DB",
        "/Desktop DF",
        "/Network/Servers",
        "/Previous Systems",
        "/Users/Shared/SC Info",
        "/Users/Guest",
        "/dev",
        "/home",
        "/net",
        "/private/var/db/efw_cache",
        "/private/var/db/Spotlight",
        "/private/var/db/Spotlight-V100"
    );
    UserPathsExcluded =     (
        "Library/Application Support/SyncServices/data.version",
        "Library/Caches",
        "Library/Logs",
        "Library/Mail/Envelope Index",
        "Library/Mail/AvailableFeeds",
        "Library/Mirrors",
        "Library/PubSub/Database",
        "Library/PubSub/Downloads",
        "Library/PubSub/Feeds",
        "Library/Safari/Icons.db",
        "Library/Safari/WebpageIcons.db",
        "Library/Safari/HistoryIndex.sk"
    );
}